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/lib/types.ts

@@ -88,6 +88,7 @@ export interface AppSettings {
   custom_image_name: string | null;
   global_aws: GlobalAwsSettings;
   global_claude_instructions: string | null;
+  global_custom_env_vars: EnvVar[];
   auto_check_updates: boolean;
   dismissed_update_version: string | null;
 }