design(web): "Tactile Deck" redesign with light/dark theming

- Full semantic design-token layer (surfaces, spacing scale, radii, elevation,
  type scale) as the single source of truth; electric-violet signature accent
  instead of default blue.
- Light + dark themes via prefers-color-scheme; muted text now meets WCAG AA
  (replaces low-contrast #aaa/#888).
- Macro cards read as physical keys (raised, hairline highlight, per-macro
  --macro-color wash + placeholder, press/executing feedback, 2-line clamp).
- Proper modal sheet (blurred scrim, sticky header/footer, focus rings),
  thumb-reachable floating action button, 44px tap targets, app-wide
  :focus-visible, and prefers-reduced-motion support.
- manifest + theme-color meta updated to the new palette.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/manifest.json

@@ -6,9 +6,10 @@
     "start_url": "/",
     "scope": "/",
     "display": "standalone",
-    "background_color": "#2e2e2e",
-    "theme_color": "#007acc",
+    "background_color": "#15161a",
+    "theme_color": "#7c5cff",
     "orientation": "any",
+    "//": "Maskable icons ideally need ~10% transparent padding so they aren't cropped inside the platform safe zone; the current PNGs are reused as-is without new art.",
     "icons": [
         {
             "src": "/static/icons/icon-192.png",