feat(frontend): cross-tab request history via broadcast module

Motivation

With one-worktree-per-branch workflows, several app tabs are open at once — and each toolbar only sees its own tab's requests. The broadcast module in @zappzarapp/browser-utils provides the cross-tab transport ready-made.

Scope

• Publish new history entries on a BroadcastChannel.
• Merge remote entries into the history panel, visibly marked with their origin (tab / branch label — the branch is already resolved per request).
• Opt-in setting in the settings dialog, default off.
• Storage stays per-tab; builds on the migrated StorageManager (#28).

Acceptance criteria

• Requests from a second tab appear in the first tab's history, marked as remote.
• Toggling the setting off stops both publishing and receiving.
• No cross-origin leakage: BroadcastChannel is same-origin by design; channel naming and this boundary documented.
• Frontend tests cover publish/receive/merge.

Start by locating the frontend history panel, settings dialog, and StorageManager migration from #28, then trace how the @zappzarapp/browser-utils broadcast module is exposed. Done means the acceptance criteria pass, including remote-entry marking, opt-out behavior, same-origin/channel documentation, and frontend publish/receive/merge tests.