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

Kanban/UI/Card/CardWindowMetrics.swift

@@ -68,10 +68,12 @@ enum CardWindowMetrics {
 
     /// What a sidebar *section* actually gets to lay out in: the column minus its two gutters.
     ///
-    /// Named because one section needs a number rather than a proposal — the embedded style editor's
-    /// grids are a fixed count of fixed-size wells per row, and the count has to be decided before
-    /// the layout runs (`StyleEditorLayout.sidebar(contentWidth:)`). Everything else in the sidebar
-    /// simply fills what it is proposed and never asks.
+    /// Named because a section can need a number rather than a proposal — originally the embedded
+    /// style editor's grids, a fixed count of fixed-size wells per row whose count had to be decided
+    /// before the layout ran (`StyleEditorLayout.sidebar(contentWidth:)`); the Style section's compact
+    /// `SymbolPicker` (2026-08-08) replaced that embed, but the Background color combo it sits beside
+    /// still sizes itself off this figure (`CardStyleSection.backgroundComboRow`). Everything else in
+    /// the sidebar simply fills what it is proposed and never asks.
     static func sidebarContentWidth(bodyPointSize: CGFloat) -> CGFloat {
         sidebarWidth(bodyPointSize: bodyPointSize) - 2 * gutter(bodyPointSize: bodyPointSize)
     }