chore: harden test isolation, gitignore stale src/data, mark shutdown action done

- ``tests/test_preferences_api.py`` no longer captures the auth API
  key at module-import time. The new ``client`` fixture resolves it
  inside its body and bakes the Bearer header into ``TestClient.headers``,
  so the e2e conftest swapping the global config singleton during
  collection cannot leave the test holding a stale 401-bound header.
  Same proven pattern as ``test_audio_processing_templates_api.py``.
- ``.gitignore`` now anchors ``/server/src/data/`` defensively. If the
  server is launched from ``server/src/`` (uncommon but possible during
  ad-hoc debugging), its relative ``data/`` resolves there. Templates
  now live in SQLite (``capture_templates`` / ``pattern_templates`` /
  ``postprocessing_templates`` tables); any stale ``*.json`` that
  lands in that directory is a runtime export and must not be
  committed.
- Three such stale exports were untracked at the start of the
  pre-merge audit and have been deleted from the working tree.
- ``TODO.md`` flips the shutdown-action checklist to done and notes
  that real-hardware verification (WLED + serial after Ctrl+C) is
  still pending.

TODO.md

@@ -1,5 +1,19 @@
 # LedGrab TODO
 
+## Server shutdown action
+
+Let user choose what happens to LED targets on server shutdown.
+
+- [x] Backend storage: `shutdown_action` in `db.settings` (`"stop_targets"` default | `"nothing"`)
+- [x] Backend route: `GET/PUT /api/v1/system/shutdown-action` in `system_settings.py`
+- [x] Backend schema: `ShutdownActionResponse/Request` in `schemas/system.py`
+- [x] Backend wiring: lifespan shutdown in `main.py` reads action, passes `restore_devices` flag to `processor_manager.stop_all()`
+- [x] `processor_manager.stop_all(restore_devices: bool = True)` — when False, calls public `proc.cancel_task()` (defined on `TargetProcessor`) which awaits cancellation without restoring device state; skips `_restore_device_idle_state` loop. No reach into private `_task` attribute.
+- [x] Frontend: hidden `<select>` + IconSelect in `settings.html` General tab (icons via `ICON_SQUARE` / `ICON_CIRCLE` from `core/icons.ts`)
+- [x] Frontend: load/save handlers in `features/settings.ts`, wired into `openSettingsModal()`
+- [x] i18n: en / ru / zh keys for label, hint, item descriptions
+- [ ] Real-hardware test pending — verify that "nothing" actually leaves a WLED + a serial device on the last frame after `Ctrl+C`/SIGTERM.
+
 ## WebUI Redesign — "Lumenworks" Studio-Console Aesthetic
 
 Full-app UI/UX refresh. Design direction committed to by user 2026-04-24.