Add server-side folder picker

New GET /api/browse endpoint lists subdirectories at any path.
UI gets a folder icon button next to each path input that opens
a browsable directory tree modal. Escape or Cancel closes it,
clicking a folder navigates into it, Select confirms the choice.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/scanner.py

@@ -35,7 +35,9 @@ VIDEO_EXT = {
 
 SUPPORTED_EXT = PHOTO_EXT | VIDEO_EXT
 
-DB_PATH = "/data/dupfinder.db"
+_DATA_DIR = Path("/data") if Path("/data").exists() else Path(__file__).parent.parent / "data"
+_DATA_DIR.mkdir(parents=True, exist_ok=True)
+DB_PATH = str(_DATA_DIR / "dupfinder.db")
 
 # Shared scan state (updated by background thread, read by status endpoint)
 scan_state = {