panel: redesign web UI and make syncing responsive

Web UI (panel/frontend/index.html) rewritten:
- Cards update in place from a single /status poll instead of being rebuilt
  on every action, so open tabs, unsaved compose/route edits, logs and the
  file browser position survive refreshes. Polling speeds up while an
  operation runs and pauses in background tabs; a header indicator shows
  when the panel last synced and detects an expired Authelia session.
- New-app dialog (starter / compose / git), suggested port and domain,
  proper confirm dialogs (the old "OK = keep volumes" remove prompt is gone),
  toasts, an activity drawer with operation output, overflow menu, search,
  status filters, keyboard shortcuts, deep links, dark mode and mobile layout.
- Tabs: overview (containers + routes), compose editor (dirty tracking,
  Ctrl+S), logs with follow, validated routes editor, file browser with
  drag-and-drop upload, backups, and a git source tab (deployed commit,
  check for updates, sync & deploy).

API (panel/panel-api.py):
- ThreadingHTTPServer so a long deploy no longer blocks every other request.
- Per-app operation lock; concurrent writes to a busy app return 409.
- GET /status: all apps, routes and container status in one request
  (statuses gathered in parallel); status reports running/partial/stopped.
- Git sync is fetch + hard reset instead of pull-or-reclone, keeps the stored
  token, reports before/after commits; GET /apps/<name>/repo[?fetch=1].
- Any http(s) git host (e.g. Forgejo), default branch detection, git
  timeouts, no credential prompts, tokens redacted from errors, and manifest
  values validated before being written into the bash-sourced manifest.

panelctl:
- flock around routes.caddy rewrites (util-linux added to the service path).
- deploy returns compose output so failures are visible in the UI.
- inspect-volumes no longer fails for apps without named podman volumes,
  which broke the file browser.

Docs: README/API.md updated; fixed outdated panelctl init examples.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UbWSNkXxZhYf7eqHTyx3Bf

panel.nix

@@ -33,6 +33,7 @@
       pkgs.zip
       pkgs.unzip
       pkgs.git
+      pkgs.util-linux # flock, used by panelctl to serialise routes file writes
     ];
 
     serviceConfig = {