Release 5.1.6: master-detail accounts panel with Antigravity pool rotation

Rebuild the settings provider-connections section as master-detail (B-form): one list entry per source — the Antigravity account pool collapses into a single entry with in-pool rotation (activate/remove/add-account/auto-failover) — and a detail pane reusing the shared connection controls over read-only facts (C-001 preserved).

• Credential writes route through the host api-remotes credentials namespace (replacing the nonexistent connection.api facade).
• A pending host-side Antigravity authorization survives a remount via auth.pendingLogin() + a pending-login endpoint; pool login operations carry an operation generation, so late reattach responses and abandoned polls never override a newer login or unlock a cancel in flight.
• Sidebar footer and dock rows share one window choice (watched first, else tightest); entry-summary pools carry their official windows.
• Ollama Local stays out of the accounts panel; an emptied pool keeps the add-account flow reachable.

Supersedes v5.1.5 in-tree (OAuth lifecycle + SDK stream diagnostics included). 330 tests passing. No new real-provider claim; no data migration; no core changes. Tag commit: f2134ba