replace symbol grid in card window sidebar with a compact symbol picker

The card window sidebar's Style section embedded StyleEditorView symbols-only
(showsBackground: false, showsSymbols: true), drawing the curated grid whole
inside the sidebar's own scroll view — a permanently open 5-7 row grid ahead
of every other section. Replace it with the reusable compact SymbolPicker
(already used by BoardInfoPopover for the board's glyph): a single well at
rest, the grid only inside its own popover.

- CardStyleSection no longer instantiates StyleEditorView at all. A new
  "Symbol" row mirrors the existing "Background" row's inspector shape
  (caption leading, control trailing), wiring SymbolPicker's onSelect/
  onSelectColor to icon/iconColor through StyleCommand.apply(...on: undo),
  the exact funnel the background combo already rides — so the card window's
  undo session semantics (13-native-undo.md) are unchanged.
- The picker is fed CuratedSymbols.available rather than its own smaller
  general-purpose default, so a card's curated vocabulary doesn't shrink.
- StyleEditorLayout.sidebar and showsBackground stay in StyleEditor.swift
  (still correct, still tested) rather than being cut as dead code — a
  separate, larger cleanup this card doesn't make (recorded on the card).
- DESIGN/03-board-ui.md and DESIGN/05-card-window.md: updated the sentences
  describing the sidebar hosting the style editor's symbol grid to describe
  the compact picker instead.
- Tests: CardSessionUndoTests gains a symbol/tint analogue of the existing
  background-combo session-routing test; CardSidebarTests gains a curated-set
  tripwire and an updated file-header note.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

KanbanTests/CardSessionUndoTests.swift

@@ -109,6 +109,19 @@ private func body(_ fixture: WriterFixture, _ path: String) throws -> String {
     try FrontmatterDocument.parse(fixture.indexText(path)).body
 }
 
+/// A `StyleRecents` of this test's own — `StyleRecentsTests`' own pattern (`StyleModelTests.swift`),
+/// restated here rather than shared across files: the list is a real user preference, and a suite
+/// that wrote into `UserDefaults.standard` would be editing the developer's own quick-style row.
+@MainActor
+private func makeRecents() -> (recents: StyleRecents, teardown: () -> Void) {
+    let name = "CardSessionUndoTests-\(UUID().uuidString)"
+    guard let defaults = UserDefaults(suiteName: name) else {
+        Issue.record("could not create a defaults suite")
+        return (StyleRecents(defaults: .standard), {})
+    }
+    return (StyleRecents(defaults: defaults), { defaults.removePersistentDomain(forName: name) })
+}
+
 // MARK: - Routing
 
 @MainActor
@@ -452,6 +465,45 @@ struct CardSessionCloseTests {
         let document = try FrontmatterDocument.parse(fixture.indexText(cardPath))
         #expect(document.background.isMissing, "the session's net effect, walked back")
     }
+
+    /// The sidebar's compact `SymbolPicker` (2026-08-08, replacing the embedded curated grid) writes
+    /// through the same funnel the test above exercises for the background combo —
+    /// `StyleCommand.apply`, exactly as `CardStyleSection.applySymbol`/`applyIconColor` call it — so
+    /// this is that same claim, `icon`/`iconColor` instead of `background`. A control swap that left
+    /// this funnel unrouted would still look correct on screen (the well updates) right up until the
+    /// window closes and the change never reaches board history.
+    @Test("A symbol and tint change made in the window's sidebar joins the session, not the board")
+    func sidebarSymbolPickingIsASessionGesture() async throws {
+        let fixture = try WriterFixture()
+        defer { fixture.tearDown() }
+        _ = try makeCommentBoard(fixture)
+        let window = try makeWindow(fixture)
+        let (recents, teardown) = makeRecents()
+        defer { teardown() }
+
+        StyleCommand.apply(
+            icon: .set("flame"),
+            iconColor: .set("carnation"),
+            to: CardStyleSection.target(forCard: cardID),
+            in: window.store,
+            recents: recents,
+            on: window.window
+        )
+        #expect(window.window.stack.undoActionName == "Restyle Card")
+        #expect(!window.board.canUndo)
+        // A symbol pick is not a background pick — "only `.set` reaches `StyleRecents.record`" is
+        // `StyleCommand.apply`'s rule for `background` alone (`StyleEditor.swift`) — so the picker's
+        // glyph and tint wells never touch the quick-style row.
+        #expect(recents.backgrounds.isEmpty)
+
+        await window.session.endSession()
+        #expect(window.board.undoActionName == coarseStep)
+
+        window.board.undo()
+        let document = try FrontmatterDocument.parse(fixture.indexText(cardPath))
+        #expect(document.icon.isMissing, "the session's net effect, walked back")
+        #expect(document.iconColor.isMissing, "and its tint with it")
+    }
 }
 
 // MARK: - Transactional staleness