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/utils/PlatformServiceMixin.ts

@@ -737,6 +737,87 @@ export const PlatformServiceMixin = {
       }
     },
 
+    // =================================================
+    // SMART DELETION PATTERN DAL METHODS
+    // =================================================
+
+    /**
+     * Get all account DIDs ordered by creation date
+     * Required for smart deletion pattern
+     */
+    async $getAllAccountDids(): Promise<string[]> {
+      const result = await this.$dbQuery(
+        "SELECT did FROM accounts ORDER BY dateCreated, did",
+      );
+      return result?.values?.map((row) => row[0] as string) || [];
+    },
+
+    /**
+     * Get account DID by ID
+     * Required for smart deletion pattern
+     */
+    async $getAccountDidById(id: number): Promise<string> {
+      const result = await this.$dbQuery(
+        "SELECT did FROM accounts WHERE id = ?",
+        [id],
+      );
+      return result?.values?.[0]?.[0] as string;
+    },
+
+    /**
+     * Get active DID (returns null if none selected)
+     * Required for smart deletion pattern
+     */
+    async $getActiveDid(): Promise<string | null> {
+      const result = await this.$dbQuery(
+        "SELECT activeDid FROM active_identity WHERE id = 1",
+      );
+      return (result?.values?.[0]?.[0] as string) || null;
+    },
+
+    /**
+     * Set active DID (can be null for no selection)
+     * Required for smart deletion pattern
+     */
+    async $setActiveDid(did: string | null): Promise<void> {
+      await this.$dbExec(
+        "UPDATE active_identity SET activeDid = ?, lastUpdated = datetime('now') WHERE id = 1",
+        [did],
+      );
+    },
+
+    /**
+     * Count total accounts
+     * Required for smart deletion pattern
+     */
+    async $countAccounts(): Promise<number> {
+      const result = await this.$dbQuery("SELECT COUNT(*) FROM accounts");
+      return (result?.values?.[0]?.[0] as number) || 0;
+    },
+
+    /**
+     * Deterministic "next" picker for account selection
+     * Required for smart deletion pattern
+     */
+    $pickNextAccountDid(all: string[], current?: string): string {
+      const sorted = [...all].sort();
+      if (!current) return sorted[0];
+      const i = sorted.indexOf(current);
+      return sorted[(i + 1) % sorted.length];
+    },
+
+    /**
+     * Ensure an active account is selected (repair hook)
+     * Required for smart deletion pattern bootstrapping
+     */
+    async $ensureActiveSelected(): Promise<void> {
+      const active = await this.$getActiveDid();
+      const all = await this.$getAllAccountDids();
+      if (active === null && all.length > 0) {
+        await this.$setActiveDid(this.$pickNextAccountDid(all));
+      }
+    },
+
     // =================================================
     // ULTRA-CONCISE DATABASE METHODS (shortest names)
     // =================================================