Fix CI: add pagefind to web devDeps + portable AppImage cache path

Two regressions surfaced after the 1.2.2 release merge to main, both
in workflows that auto-trigger on push to main:

* Deploy to GitHub Pages — build failed with `pagefind: not found`
  (exit 127) after Next.js prerendered all 241 routes. pagefind was
  not declared in web/package.json; the local build only worked
  because the project root had its own package.json with pagefind
  as a devDep (the one we just gitignored in the previous commit).
  Add `pagefind: ^1.5.2` to web/package.json devDependencies and
  regenerate web/package-lock.json so `npm ci` in CI puts the
  binary at web/node_modules/.bin/pagefind.

* Build ProxMenux Monitor AppImage — failed at the first step with
  `mkdir: cannot create directory '/var/cache/proxmenux-build':
  Permission denied`. The cache path was hardcoded to /var/cache/,
  which is writable when the script runs as root (the .50 host
  manual build) but not as the unprivileged GitHub Actions runner.
  Switch to `${XDG_CACHE_HOME:-$HOME/.cache}/proxmenux-build/` —
  works identically in both environments.

Verified locally: `cd web && npm ci && npm run build` produces 2804
files in out/, 231 pages indexed by pagefind, root redirect intact.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/package.json

@@ -79,6 +79,7 @@
     "@types/node": "^22",
     "@types/react": "^18",
     "@types/react-dom": "^18",
+    "pagefind": "^1.5.2",
     "postcss": "^8",
     "tailwindcss": "^3.4.17",
     "typescript": "^5"