feat: Add 'Restore Default Themes' command for vaults missing theme structure

- New restore_default_themes() Rust fn: seeds both _themes/ and theme/ dirs
- Per-file idempotent: never overwrites existing files with content
- Fixed ensure_vault_themes() to include minimal.md (was missing)
- New 'Restore Default Themes' command in Cmd+K Appearance group
- 3 new Rust tests + 4 new frontend tests
- 1676 frontend tests passed

src-tauri/src/lib.rs

@@ -500,6 +500,12 @@ fn ensure_vault_themes(vault_path: String) -> Result<(), String> {
     theme::ensure_vault_themes(&vault_path)
 }
 
+#[tauri::command]
+fn restore_default_themes(vault_path: String) -> Result<String, String> {
+    let vault_path = expand_tilde(&vault_path);
+    theme::restore_default_themes(&vault_path)
+}
+
 fn log_startup_result(label: &str, result: Result<usize, String>) {
     match result {
         Ok(n) if n > 0 => log::info!("{}: {} files", label, n),
@@ -699,7 +705,8 @@ pub fn run() {
             set_active_theme,
             create_theme,
             create_vault_theme,
-            ensure_vault_themes
+            ensure_vault_themes,
+            restore_default_themes
         ])
         .build(tauri::generate_context!())
         .expect("error while building tauri application")