refactor: enforce device labels, unify screenshot, remove deprecated commands, session-id-less design

- Device labels: lowercase, no whitespace, only a-z 0-9 - _ (enforced at config time)
- Session IDs removed: device label is the sole identifier
- Routes changed: /sessions/:id → /devices/:label
- Removed commands: click, type, find-window, wait-for-window, label, old version, server-version
- Renamed: status → version (compares relay/remote.py/client commits)
- Unified screenshot: takes 'screen' or a window label as argument
- Windows listed with human-readable labels (same format as device labels)
- Single instance enforcement via PID lock file
- Removed input.rs (click/type functionality)
- All docs and code in English
- Protocol: Hello.label is now required (String, not Option<String>)
- Client auto-migrates invalid labels on startup

crates/client/Cargo.toml

@@ -24,6 +24,7 @@ base64 = "0.22"
 png = "0.17"
 futures-util = "0.3"
 colored = "2"
+scopeguard = "1"
 terminal_size = "0.3"
 unicode-width = "0.1"