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/models/app_settings.rs

@@ -1,9 +1,15 @@
 use serde::{Deserialize, Serialize};
 
+use super::project::EnvVar;
+
 fn default_true() -> bool {
     true
 }
 
+fn default_global_instructions() -> Option<String> {
+    Some("If the project is not initialized with git, recommend to the user to initialize and use git to track changes. This makes it easier to revert should something break.".to_string())
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum ImageSource {
@@ -54,8 +60,10 @@ pub struct AppSettings {
     pub custom_image_name: Option<String>,
     #[serde(default)]
     pub global_aws: GlobalAwsSettings,
-    #[serde(default)]
+    #[serde(default = "default_global_instructions")]
     pub global_claude_instructions: Option<String>,
+    #[serde(default)]
+    pub global_custom_env_vars: Vec<EnvVar>,
     #[serde(default = "default_true")]
     pub auto_check_updates: bool,
     #[serde(default)]
@@ -72,7 +80,8 @@ impl Default for AppSettings {
             image_source: ImageSource::default(),
             custom_image_name: None,
             global_aws: GlobalAwsSettings::default(),
-            global_claude_instructions: None,
+            global_claude_instructions: default_global_instructions(),
+            global_custom_env_vars: Vec::new(),
             auto_check_updates: true,
             dismissed_update_version: None,
         }