Implement visual accommodations and Full Keyboard Access

Full relative text scaling per DESIGN/10: BoardMetrics is the board
strip's geometry as a pure function of the body point size
(CardWindowMetrics' twin) — lane plate/header/band, card
corner/stripe/padding, masonry spacing, the drop model's nominal card
height, resize-handle geometry, trash hatch pitch, and both window
floors all derive from an em; CardFaceMetrics folded in. The two fixed
font sizes (welcome brand/glyph) went relative; the toolbar search
field is 17 ems like the transient bar's. The no-horizontal-scroll
invariant is pinned by test at six text sizes by twelve lane counts.

Accommodations is Motion's sibling for the visual settings: Increase
Contrast adds a flat point to strokes (monotone, hierarchy-preserving),
gives borderless card/lane plates a resting separator hairline, and
takes faded accents to full alpha; Reduce Transparency turns the
transient search bar's glass solid and does the same for the alpha
washes that composite over a user-chosen board background (trash plate,
hatched header, drag shadow). Reduce Motion audited — every animated
surface already routes through Motion with a reduced variant; no gaps.

Full Keyboard Access: the template chooser's tiles were pointer-only —
now focusable, arrow-navigable (clamped, StyleWellGrid's rule), Space
picks, Return stays the sheet's default action, focus names the
selection one-way. The board's single tab stop shows its focus ring
under FKA (focusEffectDisabled inverts). Style editor verified already
conformant. Edge accents verified text-free; trash hatch pitch now
font-derived so it still reads as hatching at large text.

1549 unit tests green, both schemes build.

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

Kanban/UI/Board/BoardSearchField.swift

@@ -447,16 +447,26 @@ struct BoardSearchBar: View {
     let store: BoardStore
     let presentation: BoardSearchPresentation
 
+    /// Reduce Transparency — **this bar is the board's one glass underlay** ("glass underlays go
+    /// solid, wherever they appear", 10-accessibility.md; the design's own example, the card face
+    /// carousel's page dots, died with the carousel). `.bar` is a material, so under the setting it
+    /// becomes the opaque window background (`Accommodations.Underlay`).
+    @Environment(\.accessibilityReduceTransparency) private var reduceTransparency
+
+    private var pointSize: CGFloat { BoardMetrics.bodyPointSize }
+
     var body: some View {
         VStack(spacing: 0) {
             HStack(spacing: 0) {
                 Spacer(minLength: 0)
                 BoardSearchFieldView(store: store, presentation: presentation)
-                    .frame(width: 220)
+                    // A field wide enough for a query, in characters rather than points, so it grows
+                    // with the text it holds (10-accessibility.md's full-relative-scaling rule).
+                    .frame(width: BoardMetrics.em(17, bodyPointSize: pointSize))
             }
-            .padding(.horizontal, 12)
-            .padding(.vertical, 6)
-            .background(.bar)
+            .padding(.horizontal, BoardMetrics.stripGap(bodyPointSize: pointSize))
+            .padding(.vertical, BoardMetrics.lanePlatePadding(bodyPointSize: pointSize))
+            .background(Accommodations.underlay(reduceTransparency: reduceTransparency).style)
 
             Divider()
         }