refactor(electrobun): simplify bun backend — extract db-utils, merge handlers

- db-utils.ts: shared openDb() (WAL, busy_timeout, foreign_keys, mkdirSync)
- 5 DB modules use openDb() instead of duplicated PRAGMA boilerplate
- bttask-db shares btmsg-db's Database handle (was duplicate connection)
- misc-handlers.ts: 14 inline handlers extracted from index.ts
- index.ts: 349→195 lines (only window controls remain inline)
- updater.ts: removed dead getLastKnownVersion()
- Net reduction: ~700 lines of duplicated boilerplate

ui-electrobun/src/bun/settings-db.ts

@@ -6,8 +6,8 @@
 
 import { Database } from "bun:sqlite";
 import { homedir } from "os";
-import { mkdirSync } from "fs";
 import { join } from "path";
+import { openDb } from "./db-utils.ts";
 
 // ── DB path ──────────────────────────────────────────────────────────────────
 
@@ -103,10 +103,7 @@ export class SettingsDb {
   private db: Database;
 
   constructor() {
-    // Ensure config dir exists before opening DB
-    mkdirSync(CONFIG_DIR, { recursive: true });
-
-    this.db = new Database(DB_PATH);
+    this.db = openDb(DB_PATH);
     this.db.exec(SCHEMA);
     this.db.exec(SEED_GROUPS);