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/src/windows_mgmt.rs

@@ -1,4 +1,4 @@
-use helios_common::protocol::WindowInfo;
+use helios_common::protocol::{sanitize_label, WindowInfo};
 
 // ── Windows implementation ──────────────────────────────────────────────────
 
@@ -14,7 +14,6 @@ mod win_impl {
         keybd_event, KEYEVENTF_KEYUP, VK_MENU,
     };
 
-    // Collect HWNDs via EnumWindows callback
     unsafe extern "system" fn enum_callback(hwnd: HWND, lparam: LPARAM) -> BOOL {
         let list = &mut *(lparam.0 as *mut Vec<HWND>);
         list.push(hwnd);
@@ -38,19 +37,29 @@ mod win_impl {
         String::from_utf16_lossy(&buf[..len as usize])
     }
 
+    /// Generate a human-readable label from a window title.
+    /// E.g. "Google Chrome" -> "google_chrome", "Discord" -> "discord"
+    fn window_label(title: &str) -> String {
+        sanitize_label(title)
+    }
+
     pub fn list_windows() -> Result<Vec<WindowInfo>, String> {
         let hwnds = get_all_hwnds();
         let mut windows = Vec::new();
         for hwnd in hwnds {
             let visible = unsafe { IsWindowVisible(hwnd).as_bool() };
             let title = hwnd_title(hwnd);
-            // Only return visible windows with a non-empty title
             if !visible || title.is_empty() {
                 continue;
             }
+            let label = window_label(&title);
+            if label.is_empty() {
+                continue;
+            }
             windows.push(WindowInfo {
                 id: hwnd.0 as u64,
                 title,
+                label,
                 visible: true,
             });
         }
@@ -71,9 +80,6 @@ mod win_impl {
         Ok(())
     }
 
-    /// Bypass Windows Focus Stealing Prevention by sending a fake Alt keypress
-    /// before calling SetForegroundWindow. Without this, SetForegroundWindow
-    /// silently fails when the calling thread is not in the foreground.
     unsafe fn force_foreground(hwnd: HWND) {
         keybd_event(VK_MENU.0 as u8, 0, Default::default(), 0);
         keybd_event(VK_MENU.0 as u8, 0, KEYEVENTF_KEYUP, 0);