Add mobile bottom nav + PWA installability

Same shell pattern as top-buh/top-finance/thbd-portal: desktop top nav
stays >=lg, a fixed bottom tab bar takes over below it. Only 2 nav
links here so both fit directly, no overflow menu needed.

Also adds manifest.webmanifest + 192/512 icons (rendered from the
existing icon.svg) and appleWebApp/viewport metadata so the app can be
installed to a phone's home screen.

Nudged ticket-toasts.tsx's fixed bottom offset so it clears the new
bottom nav bar on mobile instead of overlapping it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG

public/manifest.webmanifest

@@ -0,0 +1,14 @@
+{
+  "name": "top-tickets",
+  "short_name": "top-tickets",
+  "description": "Helpdesk for support@top-sysops.ru, Telegram, and the web.",
+  "start_url": "/dashboard",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#f8f7fb",
+  "theme_color": "#6d28d9",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" }
+  ]
+}