Add UX enhancements: modals for env vars and instructions, global env vars, taskbar icon fix

- Fix Windows taskbar icon by loading icon.ico instead of icon.png (ICO contains
  multiple sizes native to Windows taskbar/title bar/alt-tab)
- Add "Container must be stopped to change settings" warning banner in config panel
- Move per-project Environment Variables and Claude Instructions into modal dialogs
  for more editing space, with buttons in the config panel to open them
- Move global Claude Instructions into a modal in Settings panel
- Add default global Claude instruction recommending git initialization
- Add global environment variables support (full stack: Rust model, TS types,
  container creation with merge logic where project overrides global for same key,
  fingerprinting for recreation checks, and Settings UI with modal)

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

app/src-tauri/src/lib.rs

@@ -27,7 +27,7 @@ pub fn run() {
             exec_manager: ExecSessionManager::new(),
         })
         .setup(|app| {
-            let icon = tauri::image::Image::from_bytes(include_bytes!("../icons/icon.png"))
+            let icon = tauri::image::Image::from_bytes(include_bytes!("../icons/icon.ico"))
                 .expect("Failed to load window icon");
             if let Some(window) = app.get_webview_window("main") {
                 let _ = window.set_icon(icon);