fix(web): prevent stale dashboard bundles

[sppidy]

What changed

• render the dashboard routes dynamically
• serve HTML with private, no-store cache semantics
• keep hashed /_next/static assets cacheable

Why

Production was serving dashboard HTML with s-maxage=31536000, allowing old client bundles to survive a deployment. The old bundle kept attempting the retired WebSocket target, so live terminal controls never reached the server even though fresh-browser terminal traffic worked end-to-end.

Verification

• npm run typecheck
• npm run lint (0 errors; existing warnings)
• npm run build
• local production response: Cache-Control: private, no-cache, no-store, max-age=0, must-revalidate
• fresh production Chrome E2E before this cache patch: focused xterm, typed a marker, observed outgoing TerminalData, incoming TerminalData, and rendered marker output

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