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

scripts/install.ps1

@@ -1,5 +1,5 @@
 # Helios Remote - Install Script
-# Downloads and starts the helios-remote Windows client
+# Downloads and starts the helios-remote Windows client.
 #
 # IMPORTANT: Windows Defender may block the executable.
 # If the download or launch fails, disable real-time protection first (run as Admin):
@@ -14,7 +14,6 @@ $dest = "$env:USERPROFILE\Desktop\Helios Remote.exe"
 
 Write-Host "Downloading helios-remote client..."
 
-# Try curl.exe first (built-in on Windows 10+), fall back to WebClient
 if (Get-Command curl.exe -ErrorAction SilentlyContinue) {
     curl.exe -L -o $dest $url
 } else {