Add performance pressure diagnostics / 新增性能卡顿诊断上报

[SivanCola]

Follow-up to #4285.
Related: #4318 addresses one known streaming UI jank path; this PR adds diagnostics for future high-resource or responsiveness issues.

Summary

• Add a user-initiated performance-pressure report prompt when the desktop observes long tasks, event-loop lag, or high JS heap pressure.
• Capture coarse diagnostic context such as timing, heap usage, visibility, network state, recent breadcrumbs, app version, and OS, then route it through the existing report scrubber and crash-report pipeline as kind: performance.
• Allow the crash-report Worker and dashboard to ingest and group performance diagnostics without a database schema migration.

Cache behavior

• Does not change system prompts, tool schemas, tool lists, provider request serialization, compaction behavior, or model-visible context prefixes.

CI follow-up

• Fixed the desktop CI frontend build failure by replacing the calculated z-index with a named --z-performance-report-prompt token accepted by the CSS z-index guard.

Verification

• pnpm --dir desktop/frontend build
• pnpm --dir desktop/frontend exec tsx src/__tests__/crash-reporting.test.ts
• (cd desktop && go test . -run 'TestCrashReport|TestPostCrashReport|TestScrub')
• npm --prefix workers/crash-report run typecheck
• git diff --check

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[SivanCola]

approve