Migrate frontend from Polymer to React + Vite

[Copilot]

Replaces the legacy Polymer-based UI with a modern React + Vite frontend. All schedule views, dialogs, popups, and the login page are reimplemented as React components.

Frontend build

• Vite project in frontend/ builds to static/dist/assets/ with deterministic filenames (no content hashes)
• Two entry points: index (main app) and login (sign-in page)
• Built output committed to repo; deploy workflow also runs npm run build-frontend before deploying

Flask template integration

• templates/index.html and templates/login.html stripped down to minimal loaders that reference the Vite bundles
• Server-side data injected via window.APP_DATA before the module script loads:

<script>
  window.APP_DATA = {
    userSchedule: {{ schedule | safe }},
    days: {{ days | safe }},
    lunches: {{ lunches | tojson }},
    termStarts: {{ term_starts | safe }},
    latestCommits: {{ latest_commits | safe }},
    username: {{ username | tojson }},
    version: {{ version | tojson }},
    sharePhoto: {{ share_photo }},
    isAdmin: {{ admin | tojson }},
  };
</script>

Dependencies

• firebase bumped 10.13.2 → 11.9.1 (resolves undici CVEs)
• vite bumped 5.4.8 → 6.3.7 (resolves esbuild dev-server CVE)
• static/dist/ and frontend/node_modules/ added to .prettierignore

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