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/commands/project_commands.rs

@@ -170,6 +170,7 @@ pub async fn start_project_container(
                     &existing_id,
                     &project,
                     settings.global_claude_instructions.as_deref(),
+                    &settings.global_custom_env_vars,
                 )
                 .await
                 .unwrap_or(false);
@@ -185,6 +186,7 @@ pub async fn start_project_container(
                     aws_config_path.as_deref(),
                     &settings.global_aws,
                     settings.global_claude_instructions.as_deref(),
+                    &settings.global_custom_env_vars,
                 ).await?;
                 docker::start_container(&new_id).await?;
                 new_id
@@ -201,6 +203,7 @@ pub async fn start_project_container(
                 aws_config_path.as_deref(),
                 &settings.global_aws,
                 settings.global_claude_instructions.as_deref(),
+                &settings.global_custom_env_vars,
             ).await?;
             docker::start_container(&new_id).await?;
             new_id