Build the attachments sidebar section

The card's complete file inventory: compact QuickLook-thumbnail rows
over Card.attachments — no reference tracking, subfolders tolerated
and unsurfaced — with a quiet header add affordance and the drop hint
empty state. The whole window is the file-drop surface, Edit mode
included (the editor's drag types were already filtered; now tested),
sharing the board's folder-refusal semantics literally: FinderDrop
moved verbatim into its own file so both windows run the same
partition and loss row. Dragged text still lands at the caret and is
inert elsewhere — the window delegate accepts file payloads only.
Rows open on double-click or Return, drag out their file URL, and
Remove is a bracketed write through FileManager.trashItem — the system
Trash, never a hard delete, returning the in-Trash URL so the promise
is testable; the attachment listing is the guard, so traversal and
subfolder names refuse in one line. Keyboard-native per 05: the
section is one Tab stop, arrows walk rows by name, Space toggles the
shared QuickLook panel, Backspace removes. File > Add Attachment
(shift-cmd-A) comes alive through the same import path.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/Storage/BoardWriter.swift

@@ -1539,6 +1539,67 @@ public enum BoardWriter: Sendable {
         return BoardLoader.attachmentNames(in: cardFolder)
     }
 
+    /// Moves one of a card's attachments to the **system** Trash — the attachment row's Remove and
+    /// its ⌫ twin (05-card-window.md ▸ Attachments: "context menu Open / Reveal in Finder / Remove
+    /// (moves to the **system** Trash, never hard-deletes …)").
+    ///
+    /// ### `trashItem`, and never `removeItem`
+    ///
+    /// The design says *system* Trash and means it: a board's own tombstone quasi-lane is a different
+    /// trash with a different vocabulary (03-board-ui.md § Trash's naming constraint), and an
+    /// attachment removed here is recoverable exactly the way a file dragged out of a Finder window
+    /// is — by the user, in Finder, with no help from this app. `FileManager.trashItem` is that
+    /// sentence; `removeItem` would be a hard delete of the user's own file, which this app never
+    /// does to an attachment.
+    ///
+    /// ### The listing is the guard
+    ///
+    /// `name` is checked against `BoardLoader.attachmentNames(in:)` — **the very set the sidebar
+    /// shows** — before anything is touched, which is what makes the whole class of "the caller
+    /// passed something else" unreachable in one line rather than four: a path (`../index.md`), a
+    /// subfolder, a hidden file, a symlink, and the empty string are all simply not in the listing,
+    /// and none of them can be trashed through this call. It is also `relocateLooseFiles`' re-check
+    /// rule applied here — the caller's name is re-read against disk at write time, not trusted from
+    /// whenever the row was drawn.
+    ///
+    /// ### A name that is no longer there is not a failure
+    ///
+    /// It returns `false` and writes nothing, `relocateLooseFiles`' rule again: "the reload is the
+    /// authority on what is there, and a file the user deleted between the walk and the write is not
+    /// a failure to report". A ⌫ racing an external delete of the same file is exactly that race, and
+    /// a banner for it would name a removal the user got anyway.
+    ///
+    /// - Returns: where the file now sits **inside the Trash**, or `nil` when there was nothing to
+    ///   move. The URL is returned rather than discarded because it is the only proof this call
+    ///   makes that the file still exists at all — the difference between the promise ("never
+    ///   hard-deletes") and a `removeItem` that would look identical from `attachments/`.
+    @discardableResult
+    public static func removeAttachment(
+        named name: String,
+        fromCard cardFolder: URL
+    ) throws(BoardWriteError) -> URL? {
+        let operation = WriteOperation.removeAttachment(filename: name)
+        try checkIsDirectory(cardFolder, describedAs: "card folder", operation: operation)
+        try checkIsUUIDShaped(cardFolder, operation: operation)
+
+        guard BoardLoader.attachmentNames(in: cardFolder).contains(name) else { return nil }
+
+        let fileURL = cardFolder
+            .appendingPathComponent(attachmentsFolderName, isDirectory: true)
+            .appendingPathComponent(name)
+        var trashedURL: NSURL?
+        do {
+            try FileManager.default.trashItem(at: fileURL, resultingItemURL: &trashedURL)
+        } catch {
+            throw BoardWriteError(
+                operation: operation,
+                path: fileURL.path,
+                reason: .io(message: "could not move file to the Trash: \(error.localizedDescription)")
+            )
+        }
+        return trashedURL as URL?
+    }
+
     // MARK: - Move/copy pre-flight
 
     /// The rank a moved or copied root lands on: the caller's explicit value — a drop between
@@ -1725,6 +1786,18 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
 
     case importAttachment(filename: String)
     case listAttachments
+
+    /// An attachment being moved to the **system** Trash — the card window's attachment Remove and
+    /// its ⌫ twin (05-card-window.md ▸ Attachments).
+    ///
+    /// Its own case rather than a fold into `.delete`, on the vocabulary's standing reasoning and
+    /// then some: `.delete` is the board's *tombstone*, and this app has two trashes on purpose —
+    /// "Finder's 'Move to Trash' phrasing is reserved for the system Trash; board deletion says
+    /// 'Delete'" (03-board-ui.md § Trash). A banner saying the app "couldn't delete 'shot.png'"
+    /// would claim the board's own trash took a hand in a file only Finder can give back.
+    /// `filename` is the attachment's name as the sidebar shows it.
+    case removeAttachment(filename: String)
+
     case renumberChildren           // order-maintenance sweep (compaction)
 
     /// A loose file being moved out of a card folder into its `attachments/` — the loose-file
@@ -1771,9 +1844,10 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
     case rawSource(title: String?)
 
     /// Fills in the title once the Writer has read it off the document the operation is acting
-    /// on — identity for the six cases with no title slot at all: `createBoard`/`createLane`/
-    /// `createCard` are minting a file, not reading one; `importAttachment`'s "title" is the
-    /// filename it already carries; `listAttachments` and `renumberChildren` name no single item.
+    /// on — identity for the cases with no title slot at all: `createBoard`/`createLane`/
+    /// `createCard` are minting a file, not reading one; `importAttachment`, `removeAttachment` and
+    /// `relocateLooseFile` carry a filename, which is the name the user is looking at and the only
+    /// one their banner should say; `listAttachments` and `renumberChildren` name no single item.
     /// Called once, right where the operation's `readDocument` succeeds — `updateIndex` itself
     /// (which covers every case that funnels through it: renumber, delete, restore, style, and
     /// the tail end of move/copy) and the move/copy pre-flight, before the folder travels or the
@@ -1782,7 +1856,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
     public func withTitle(_ title: String?) -> WriteOperation {
         switch self {
         case .createBoard, .createLane, .createCard, .importAttachment, .listAttachments,
-             .renumberChildren, .relocateLooseFile:
+             .removeAttachment, .renumberChildren, .relocateLooseFile:
             self
         case .move: .move(title: title)
         case .reorder: .reorder(title: title)
@@ -1823,6 +1897,7 @@ public enum WriteOperation: Sendable, Equatable, CustomStringConvertible {
         case let .duplicateBoard(title): Self.phrase("duplicate board", title)
         case let .importAttachment(filename): "import attachment '\(filename)'"
         case .listAttachments: "list attachments"
+        case let .removeAttachment(filename): "move attachment '\(filename)' to the Trash"
         case .renumberChildren: "renumber children"
         case let .relocateLooseFile(filename): "relocate loose file '\(filename)'"
         case let .toggleTask(title): Self.phrase("toggle a checkbox in", title)