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/App/BoardWindowHost.swift

@@ -72,7 +72,14 @@ struct BoardWindowHost: View {
 
     var body: some View {
         content
-            .frame(minWidth: 640, minHeight: 400)
+            // Font-derived like everything else the board lays out (`BoardMetrics.windowMinimumSize`,
+            // 10-accessibility.md's full-relative-scaling rule): at a large system text size a
+            // 640×400 floor would be narrower than two lane headers, and "every lane is always on
+            // screen" would degrade into a strip of truncation.
+            .frame(
+                minWidth: BoardMetrics.windowMinimumSize(bodyPointSize: BoardMetrics.bodyPointSize).width,
+                minHeight: BoardMetrics.windowMinimumSize(bodyPointSize: BoardMetrics.bodyPointSize).height
+            )
             .background(WindowAccessor(controller: windowController))
             .navigationTitle(windowTitle)
             .task { await start() }