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/LaneResizeHandle.swift

@@ -36,11 +36,14 @@ struct LaneResizeHandle: View {
     /// cursor.
     @State private var cursorPushed = false
 
-    private let handleWidth: CGFloat = 12
+    /// The grab strip's width and its rightward shift — both font-derived, because the inter-lane
+    /// gap they are proportioned against is (`BoardMetrics.stripGap`, 10-accessibility.md's
+    /// full-relative-scaling rule). At the standard body size they are the 12pt and 8pt the strip
+    /// has always used: with the strip trailing-aligned, +8 leaves 4pt over the lane and hangs 8pt
+    /// into the gap (clear of the lane's own scrollbar).
+    private var handleWidth: CGFloat { BoardMetrics.resizeHandleWidth(bodyPointSize: BoardMetrics.bodyPointSize) }
 
-    /// Rightward shift: with the strip trailing-aligned, +8 leaves 4pt over the lane and hangs 8pt
-    /// into the gap (clear of the scrollbar).
-    private let overhang: CGFloat = 8
+    private var overhang: CGFloat { BoardMetrics.resizeHandleOverhang(bodyPointSize: BoardMetrics.bodyPointSize) }
 
     var body: some View {
         Color.clear