fix: comprehensive security, bug, performance, and UI/UX audit

Security
- Default bind 127.0.0.1; first-run bootstrap generates random api_token
  and refuses to bind non-loopback without auth unless explicitly opted in
- Path-traversal hardened: BrowserService.validate_path rejects absolute
  paths, drive letters, UNC, NUL bytes. /api/browser/{play,metadata,
  thumbnail} now require folder_id and a folder-relative path
- Pydantic validators on links: http(s) URLs only, mdi:<slug> icons only
- Scripts/callbacks/links create/update/delete gated by *_management flags
- Strict CSP, X-Frame-Options DENY, Referrer-Policy no-referrer,
  X-Content-Type-Options nosniff
- CORS locked to localhost:<port> + 127.0.0.1:<port> by default; configurable
- config.yaml writes atomic (tmp + os.replace) and 0o600 on POSIX
- Subprocesses spawned in their own process group / new session so timeout
  kills the whole tree (Windows CREATE_NEW_PROCESS_GROUP, POSIX
  start_new_session=True)
- Frontend XSS: monitor name + details escapeHtml'd; power button moved to
  delegated data-action handler; remote MDI SVGs parsed and sanitized
  (strip script/foreignObject/on*/javascript: hrefs) before innerHTML
- All dynamic URL segments now wrapped in encodeURIComponent

Bugs
- WebSocket reconnect: close previous socket before opening new, clear
  ping interval per-socket, clear reconnectTimeout up-front, retry on
  online/visibilitychange, try/catch JSON.parse
- Artwork fetch race: AbortController + generation guard
- _broadcast_after_open: initialize status, swallow per-poll errors,
  background tasks tracked in a strong-ref set with done-callback cleanup
- Audio analyzer: sticky _unavailable flag prevents infinite start/stop
  spin when no loopback device exists; cleared by set_device()
- Volume short-circuit cache invalidated when server reports remote volume
- Browser thumbnail race: per-folder generation counter + isConnected
  checks; aborts in-flight fetches on navigation
- Track-skip uses cached title instead of full WinRT status round-trip

Performance
- Linux MPRIS/pactl and /api/display DDC-CI handlers wrapped in
  asyncio.to_thread so blocking IO never stalls the event loop
- browse_directory moved off the event loop (SMB shares could freeze it)
- Windows status poll caches one asyncio loop per worker thread via
  threading.local instead of new_event_loop/close on every 0.5s tick
- broadcast() serializes JSON once and uses send_text to all clients
- Hourly thumbnail cache cleanup scheduled in lifespan (was never invoked
  — cache grew unbounded)
- Progress drag listeners attached only while dragging

Quality
- All asyncio.get_event_loop() in coroutines → get_running_loop()
- ThreadPoolExecutors shut down cleanly during lifespan teardown
- config_manager dedup: 12 near-identical methods collapsed onto generic
  _upsert/_delete helpers (~290 lines removed)
- Service worker no longer pass-throughs every fetch
- M3U playlist written via NamedTemporaryFile (no fixed-path symlink
  clobber race)
- __version__ now prefers live pyproject.toml in dev checkouts so
  pip install -e . users see the source-of-truth version, not the stale
  package-metadata version baked in at install time

UI/UX (Studio Reference)
- Green leftover focus rings (rgba(29,185,84,...)) all replaced with
  copper accent (rgba(var(--copper-rgb),...))
- Dialogs: square corners, copper top hairline, unified with editorial
  chrome
- .browser-item: transparent with copper hover border (was filled card)
- Audio device select uses var(--sans) instead of generic system font
- Mobile container padding tuned for ≤480px screens
- Breadcrumb home is a real <button> with aria-label; aria-current on root
- i18n: filled display.msg.power_*, execution.*, scripts.params.execute,
  callbacks.empty in both en + ru

media_server/services/browser_service.py

@@ -63,14 +63,28 @@ class BrowserService:
         if not base_path.is_dir():
             raise ValueError(f"Media folder path is not a directory: {base_path}")
 
-        # Handle relative vs absolute paths
-        if requested_path.startswith("/") or requested_path.startswith("\\"):
-            # Relative to folder root (remove leading slash)
-            requested_path = requested_path.lstrip("/\\")
+        # Reject absolute paths, drive letters, UNC paths, and NUL bytes outright.
+        # Only true folder-relative paths are accepted.
+        if "\x00" in requested_path:
+            raise ValueError("Path contains NUL byte")
+
+        # Strip a single leading "/" or "\\" (legacy callers send "/sub/dir") but
+        # then refuse anything that still looks absolute.
+        cleaned = requested_path.lstrip("/\\")
+        # Detect Windows drive letter like "C:/..." after stripping.
+        if len(cleaned) >= 2 and cleaned[1] == ":":
+            raise ValueError("Absolute paths are not allowed")
+        # Detect raw UNC ("\\\\server\\share") — the lstrip above strips at most
+        # one leading slash, so a UNC original starts with another "\\" or "/".
+        if cleaned.startswith("\\") or cleaned.startswith("/"):
+            raise ValueError("Absolute paths are not allowed")
+        candidate = Path(cleaned) if cleaned else None
+        if candidate is not None and candidate.is_absolute():
+            raise ValueError("Absolute paths are not allowed")
 
         # Build and resolve full path
-        if requested_path:
-            full_path = (base_path / requested_path).resolve()
+        if cleaned:
+            full_path = (base_path / cleaned).resolve()
         else:
             full_path = base_path