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/UI/Card/CardWindowMetrics.swift

@@ -79,6 +79,27 @@ enum CardWindowMetrics {
         columnWidth(characters: bodyMinimumCharacters, bodyPointSize: bodyPointSize)
     }
 
+    // MARK: - The attachments section
+
+    /// An attachment row's thumbnail: a **small** square, one and a half ems on a side
+    /// (05-card-window.md ▸ Attachments: "Compact rows: small QuickLook thumbnail (Finder-icon
+    /// fallback) + middle-truncated filename, one row per file").
+    ///
+    /// Derived rather than a point size, like everything else here, and deliberately *small*: this
+    /// is an inventory, not a gallery — "viewing media is the card window's job" was settled about
+    /// the window, not about this list, and a row tall enough to see a screenshot in would push the
+    /// four sections beneath it off the sidebar.
+    static func attachmentThumbnailSide(bodyPointSize: CGFloat) -> CGFloat {
+        (bodyPointSize * 1.5).rounded()
+    }
+
+    /// The inset inside an attachment row, and the gap between its thumbnail and its filename —
+    /// half a gutter, the rendered body's rhythm, so the sidebar's list and the body's blocks are
+    /// spaced by the same unit.
+    static func attachmentRowPadding(bodyPointSize: CGFloat) -> CGFloat {
+        previewPadding(bodyPointSize: bodyPointSize)
+    }
+
     // MARK: - The rendered body
 
     /// One step of structural indent in Preview — a list level, a quote level. One and a half ems,