feat(server): serve dashboard SPA with deep-link fallback; remove v1 portal

Axum now serves the v2 React/Vite dashboard SPA at / with a client-side
routing fallback, and the dead v1 HTML portal is removed (nothing was
live on the server to preserve).

- SPA served from server/static/app via ServeDir with a fallback to
  index.html, so deep links (/machines, /sessions) resolve to the SPA.
- /api/*rest and /ws/*rest return JSON 404 so unrouted API/WS paths never
  leak index.html to clients; real /api, /ws, /health, /metrics, and the
  /downloads nest keep precedence (matchit static-over-wildcard).
- Path-aware Cache-Control: hashed /assets immutable, index.html no-cache.
- Vite builds to server/static/app (base /); the artifact is gitignored
  and rebuilt at deploy time (npm ci && npm run build).
- Removed v1 portal files (login/dashboard/users/index/viewer .html) and
  their dead serve_* handlers; the SPA owns /, /login, /dashboard, /users.

Verified locally: server boots, / and deep links serve the SPA, unknown
/api path returns JSON 404 (not HTML), /health and /downloads intact.
cargo build + clippy -D warnings green.

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

.gitignore

@@ -26,3 +26,6 @@ vendor/
 
 # Generated files
 *.generated.*
+
+# Built SPA (Vite build output served by the server; rebuilt from dashboard/)
+/server/static/app/