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/telemetry.ts

@@ -25,7 +25,7 @@ interface ActiveSpan {
 
 // ── Telemetry Manager ──────────────────────────────────────────────────────
 
-class TelemetryManager {
+export class TelemetryManager {
   private enabled = false;
   private endpoint = "";
   private activeSpans = new Map<string, ActiveSpan>();