feat: implement Active Pointer + Smart Deletion Pattern for accounts

- Consolidate migrations: merge 002/003 into 001_initial with UNIQUE did constraint
- Add foreign key: active_identity.activeDid REFERENCES accounts.did ON DELETE RESTRICT
- Replace empty string defaults with NULL for proper empty state handling
- Implement atomic smart deletion with auto-switch logic in IdentitySwitcherView
- Add DAL methods: $getAllAccountDids, $getActiveDid, $setActiveDid, $pickNextAccountDid
- Add migration bootstrapping to auto-select first account if none selected
- Block deletion of last remaining account with user notification

Refs: doc/active-pointer-smart-deletion-pattern.md

src/views/IdentitySwitcherView.vue

@@ -272,15 +272,48 @@ export default class IdentitySwitcherView extends Vue {
     this.notify.confirm(
       NOTIFY_DELETE_IDENTITY_CONFIRM.text,
       async () => {
-        await this.$exec(`DELETE FROM accounts WHERE id = ?`, [id]);
-        this.otherIdentities = this.otherIdentities.filter(
-          (ident) => ident.id !== id,
-        );
+        await this.smartDeleteAccount(id);
       },
       -1,
     );
   }
 
+  /**
+   * Smart deletion with atomic transaction and last account protection
+   * Follows the Active Pointer + Smart Deletion Pattern
+   */
+  async smartDeleteAccount(id: string) {
+    await this.$withTransaction(async () => {
+      const total = await this.$countAccounts();
+      if (total <= 1) {
+        this.notify.warning(
+          "Cannot delete the last account. Keep at least one.",
+        );
+        throw new Error("blocked:last-item");
+      }
+
+      const accountDid = await this.$getAccountDidById(parseInt(id));
+      const activeDid = await this.$getActiveDid();
+
+      if (activeDid === accountDid) {
+        const allDids = await this.$getAllAccountDids();
+        const nextDid = this.$pickNextAccountDid(
+          allDids.filter((d) => d !== accountDid),
+          accountDid,
+        );
+        await this.$setActiveDid(nextDid);
+        this.notify.success(`Switched active to ${nextDid} before deletion.`);
+      }
+
+      await this.$exec("DELETE FROM accounts WHERE id = ?", [id]);
+    });
+
+    // Update UI
+    this.otherIdentities = this.otherIdentities.filter(
+      (ident) => ident.id !== id,
+    );
+  }
+
   notifyCannotDelete() {
     this.notify.warning(
       NOTIFY_CANNOT_DELETE_ACTIVE_IDENTITY.message,