perf(web-ui): defer inactive session metadata startup

[limityan]

Summary

• Load initial session metadata immediately only for the visible active workspace during startup.
• Defer non-active workspace session metadata until the startup overlay handoff, with a bounded 0-120 ms stagger and a late-mount next-paint path so sections do not wait for the 10 s fallback after the overlay event has already fired.
• Make the performance E2E session lookup expand the relevant visible session sections without clicking show less, so older perf fixtures remain reachable in multi-workspace nav state.

Refs #949

Performance data

Environment: Windows desktop release-fast performance E2E on the same local workspace. Current-PR before sample is the pre-change run on gcwing/main/this branch base; after samples are the latest three current-commit release-fast runs.

Scenario	Before current PR	After current PR	Change
Cold startup to interactive_shell_ready	997.1 ms	817.5 ms median	-179.6 ms (-18.0%)
Main-window-shown to interactive	165.6 ms	142.2 ms median	-23.4 ms (-14.1%)
Before-interactive API total duration	930.9 ms	426.6 ms median	-504.3 ms (-54.2%)
Before-interactive list_persisted_sessions_page calls	5 calls	1 call in all latest samples	-4 calls

Large-session opening is kept as a regression guard in this PR, not claimed as the main optimization target because the product change is startup metadata scheduling. Latest current-commit guard results:

Scenario	Latest current-commit result	Notes
First open large history session, latest text readable	377.3 ms median	Latest 3 range: 308.9-662.6 ms; the 662.6 ms sample is retained as an outlier to keep investigating.
First open large history session, full history hydrated	1198.7 ms median	This remains a residual issue and is tracked in #949.
Warm reopen large history session, latest text readable	262.4 ms median	Latest 3 range: 259.1-306.1 ms.
Loading / blank guard while opening large history session	loading toggles=0, blank samples=0	Latest samples still record one pre-interaction scroll stabilization event.

Risk and behavior impact

• Current active workspace behavior is preserved: its visible session metadata still loads immediately.
• Non-active visible workspace sections now load metadata after startup overlay handoff. This can delay non-active session rows by one paint plus at most 120 ms after first screen handoff, but avoids putting every workspace metadata page into the startup critical window.
• Existing workspace collapse behavior is not changed; this PR does not collapse older workspaces.
• Remote workspace parameters continue to flow through the existing metadata API call; this PR only changes when the initial call is scheduled.
• The new E2E data-session-nav-toggle-action attribute is test-only metadata on the existing show-more button and does not change the user-visible control.

Verification

• pnpm --dir src/web-ui run test:run src/app/startup/startupPerformanceContract.test.ts src/app/components/NavPanel/sections/sessions/sessionMetadataStartup.test.ts
• pnpm run type-check:web
• pnpm run check:repo-hygiene
• pnpm run desktop:build:release-fast
• pnpm run e2e:test:perf:release-fast