fix(browser): auto-discover Desktop bridge on ephemeral ports

[zzalancelot]

Summary

Fixes #93 — users on DSH Desktop stay stuck on “Reconnecting…” because Desktop binds a random ephemeral port (--port 0), while the extension only probed a fixed port list and never rediscovered after the first resolve.

• Layered discovery when settings.bridgeUrl is empty: auto-cache → known CLI/Desktop ports → open loopback tabs → bounded ephemeral-port scan (49152–65535).
• BridgeClient rediscovers after 3 consecutive probe failures so Desktop restarts are followed without user action; a manual bridgeUrl always wins and is never overwritten.
• Persist successes in dshBridgeAuto (separate from settings); debounce rediscovery on loopback navigations. close(4000) → stopped is unchanged.

Test plan

• Start DSH Desktop (random port), open the side panel with empty bridge URL → reaches connected; SW log shows resolved ws://127.0.0.1:<port>/ext/bridge.
• Restart DSH (port changes) → extension follows and reconnects within ~60s without changing settings.
• Set a wrong manual bridge URL → stays on that address (no auto-correct).
• CLI dsh web on 3080 → still connects immediately (no ephemeral scan).
• pnpm --filter dsh-browser-extension run test|typecheck|build
• Another client taking the bridge (close(4000)) still leaves this client stopped.