feat: replace 'Is a' / 'is_a' with 'type' as the canonical frontmatter key

- Rust Frontmatter struct now parses both 'type' (new) and 'Is A'/'is_a' (legacy),
  with 'type' taking precedence via resolve_type()
- Added migrate_is_a_to_type() vault-wide migration (runs on startup, also exposed as Tauri command)
- Frontend: buildNoteContent and resolveNewType now emit 'type:' in YAML
- Inspector SKIP_KEYS hides 'is_a', 'Is A', 'type', 'title' from property list
  (TypeRow already renders the type with its dedicated UI)
- frontmatterToEntryPatch handles both 'type' and legacy 'is_a' keys
- Mock data updated: all YAML content uses 'type:' instead of 'is_a:'/'Is A:'
- Tests updated to expect 'type:' in generated frontmatter

Product decision: internal VaultEntry field stays as `isA` (TS) / `is_a` (Rust)
to minimize blast radius. The user-facing change is the YAML key and inspector.

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

src-tauri/src/lib.rs

@@ -100,6 +100,11 @@ fn purge_trash(vault_path: String) -> Result<Vec<String>, String> {
     vault::purge_trash(&vault_path)
 }
 
+#[tauri::command]
+fn migrate_is_a_to_type(vault_path: String) -> Result<usize, String> {
+    vault::migrate_is_a_to_type(&vault_path)
+}
+
 #[tauri::command]
 fn get_settings() -> Result<Settings, String> {
     settings::get_settings()
@@ -154,7 +159,8 @@ pub fn run() {
                 .map(|h| h.join("Laputa"))
                 .unwrap_or_default();
             if vault_path.is_dir() {
-                match vault::purge_trash(vault_path.to_str().unwrap_or_default()) {
+                let vp_str = vault_path.to_str().unwrap_or_default();
+                match vault::purge_trash(vp_str) {
                     Ok(deleted) if !deleted.is_empty() => {
                         log::info!("Purged {} trashed files on startup", deleted.len());
                     }
@@ -163,6 +169,16 @@ pub fn run() {
                     }
                     _ => {}
                 }
+                // Migrate legacy is_a/Is A frontmatter to type
+                match vault::migrate_is_a_to_type(vp_str) {
+                    Ok(n) if n > 0 => {
+                        log::info!("Migrated {} files from is_a to type on startup", n);
+                    }
+                    Err(e) => {
+                        log::warn!("Failed to migrate is_a on startup: {}", e);
+                    }
+                    _ => {}
+                }
             }
 
             Ok(())
@@ -183,6 +199,7 @@ pub fn run() {
             ai_chat,
             save_image,
             purge_trash,
+            migrate_is_a_to_type,
             get_settings,
             save_settings,
             github_list_repos,