feat: UI polish v2 + installable PWA with offline audio

Phase 1 — Vietnamese-safe font + master empty state:
- Add @fontsource/roboto-condensed (700 weight, all subsets including
  Vietnamese). font-display:swap. tan-tan-num now resolves the bundled
  face on Android instead of system Arial Narrow.
- New MasterEmptyState.svelte: ghost 11×9 grid + "Chế độ Quản trò"
  pill + readiness microcopy. Replaces the bare line of text in
  MasterPanel's no-game state.

Phase 2 — Mode picker icons, color picker layout, header polish:
- Inline SVG glyphs above each mode button (player card / megaphone /
  two stacked cards). Communicates role at a glance.
- Color picker wrapped in a single bordered card with "Tuỳ chỉnh"
  and "Mẫu sẵn" sub-headers.
- "Mặc định" → "Đặt lại" with a bordered chip style — clearer
  affordance than the previous near-invisible footer link.
- Header subline: drop tracking-[0.28em] all-caps SaaS look; replace
  with dash-flanked lantern band ("— 🏮 Hội chợ TN1 —") for
  fairground mood.

Phase 3 — Installable PWA + offline audio:
- @vite-pwa/sveltekit with autoUpdate. Precache app shell (~353 KB
  → 213 entries) PLUS the default voice's 92 clips so first-install
  is fully offline-capable. Alternate voices fall through to a
  CacheFirst runtime rule (cached on first play, 30-day TTL,
  maxEntries: 400 for future-proofing).
- New static/manifest.webmanifest (Lô tô — Hội chợ TN1, theme #1565c0,
  background #0a0f1f, standalone, vi).
- Icons: 192/512 standard + 512 maskable, generated from a single
  rose-amber-gradient SVG source.
- app.html: manifest link, dual theme-color meta (light + dark),
  apple-touch-icon, apple-mobile-web-app-capable for proper
  standalone launch on iOS Safari.
- _headers: add manifest-src + worker-src to CSP; no-cache on /sw.js
  and /manifest.webmanifest so deploys propagate.

Tests: 115/115 pass. Build clean (305 precache entries, 0 glob
warnings).

Reviewer concerns (addressed):
- maxEntries bumped 200 → 400 (was barely enough for 2 voices).
- Default voice precached so offline-first promise holds without
  requiring users to play every clip online first.
- "do NOT add skipWaiting" comment added next to autoUpdate.

web/static/_headers

@@ -1,6 +1,14 @@
 /*
-  Content-Security-Policy: default-src 'self'; img-src 'self' data:; media-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'; connect-src 'self'; font-src 'self' data:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'
+  Content-Security-Policy: default-src 'self'; img-src 'self' data:; media-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'; connect-src 'self'; font-src 'self' data:; frame-ancestors 'none'; base-uri 'self'; form-action 'self'; manifest-src 'self'; worker-src 'self'
   X-Content-Type-Options: nosniff
   Referrer-Policy: strict-origin-when-cross-origin
   Permissions-Policy: accelerometer=(), camera=(), geolocation=(), gyroscope=(), microphone=(), payment=(), usb=()
   X-Frame-Options: DENY
+
+# Service worker must always re-validate so fresh deploys propagate.
+/sw.js
+  Cache-Control: no-cache
+
+/manifest.webmanifest
+  Cache-Control: no-cache
+  Content-Type: application/manifest+json

web/static/icons/source.svg

@@ -0,0 +1,26 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" width="512" height="512">
+  <defs>
+    <linearGradient id="bg" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0%" stop-color="#f43f5e"/>
+      <stop offset="50%" stop-color="#f59e0b"/>
+      <stop offset="100%" stop-color="#f43f5e"/>
+    </linearGradient>
+    <linearGradient id="text" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#fff8e7"/>
+      <stop offset="100%" stop-color="#ffe9b8"/>
+    </linearGradient>
+  </defs>
+  <rect width="512" height="512" rx="96" fill="url(#bg)"/>
+  <rect x="56" y="56" width="400" height="400" rx="64" fill="#0a0f1f" opacity="0.18"/>
+  <text
+    x="256" y="312"
+    text-anchor="middle"
+    font-family="Roboto Condensed, Arial Narrow, system-ui, sans-serif"
+    font-weight="900"
+    font-size="240"
+    font-style="italic"
+    letter-spacing="-8"
+    fill="url(#text)"
+    style="paint-order:stroke;stroke:#0a0f1f;stroke-width:8;stroke-opacity:0.18"
+  >Lô tô</text>
+</svg>

web/static/manifest.webmanifest

@@ -0,0 +1,30 @@
+{
+  "name": "Lô tô — Hội chợ TN1",
+  "short_name": "Lô tô",
+  "description": "Bàn số của trò chơi Lô tô",
+  "start_url": ".",
+  "scope": ".",
+  "display": "standalone",
+  "orientation": "portrait",
+  "theme_color": "#1565c0",
+  "background_color": "#0a0f1f",
+  "lang": "vi",
+  "icons": [
+    {
+      "src": "icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    },
+    {
+      "src": "icons/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}