fix(ui-react): harden chat async and lazy fallbacks

[Astro-Han]

Summary

• keep chat model switching and session row actions rejection-safe by catching delegated action failures locally while their AppShell owners keep visible toast feedback
• give visible lazy chunks compact loading fallbacks, while keeping ArtifactPane lazy loading hidden until the pane itself can prove live artifacts or a current-session list error
• add/adjust renderer contracts for chat async boundaries, lazy fallback ownership, ArtifactPane hidden default, and the existing voice microphone permission probe rejection boundary

Scope

Part of #477.

Async scope in this PR is intentionally limited to:

• chat model switcher pending chrome
• sidebar session row action pending chrome

Voice Settings is not changed in production here. Its microphone permission probe already catches navigator.permissions.query() rejection and returns unknown; this PR adds a contract for that existing boundary.

User-visible impact is limited to compact loading placeholders for surfaces that are already supposed to be visible. ArtifactPane now preserves the existing default-hidden product contract while its lazy chunk loads.

Verification

• RED: lazy fallback contract failed while ArtifactPaneFallback rendered a fake .maka-artifact-pane
• RED: async boundary contract failed while model switcher and session row actions depended on the shared swallow-errors helper
• GREEN: npm run -w @maka/desktop test (1727 passed)
• GREEN: npm run -w @maka/desktop build:renderer (passes; existing Vite large chunk warning remains)
• GREEN: npm --workspace @maka/ui run build && npm run -w @maka/desktop build:main && node --test dist/main/__tests__/renderer-async-boundaries-contract.test.js dist/main/__tests__/session-sticky-model-contract.test.js dist/main/__tests__/session-row-actions-fail-soft-contract.test.js
• GREEN: npm run -w @maka/desktop build:main && node --test dist/main/__tests__/renderer-lazy-fallback-contract.test.js
• GREEN: npm run -w @maka/desktop build:main && node --test dist/main/__tests__/voice-capture-smoke-contract.test.js
• KNOWN FAIL: npm run -w @maka/ui test still fails in existing chat-primitives footer-action / lineage-badge pixel merge assertions; those files are untouched by this PR

Commit Split

• fix(ui-react): catch model switcher rejection
• fix(ui-react): catch session row action rejection
• fix(ui-react): add shell lazy fallbacks
• fix(ui-react): add module lazy fallbacks
• fix(ui-react): keep artifact pane lazy fallback hidden
• refactor(ui-react): inline async pending cleanup
• test(ui-react): pin voice permission probe rejection

[Astro-Han]

Review response pushed.

Refs #477.

Changes:

• Removed ArtifactPaneFallback; ArtifactPane Suspense is back to fallback={null} so the generated-file pane stays hidden until ArtifactPane itself has live artifact/list-error evidence.
• Reworked lazy fallback contract into visible-surface whitelist semantics: Settings/Browser/modules may reserve loading chrome; ArtifactPane must stay hidden without an explicit visibility gate.
• Removed the shared runAsyncActionBoundary swallow-errors helper; model switcher and session row pending cleanup now catch locally and rely on their AppShell action owners for visible toast feedback.
• Added a voice Settings contract confirming microphone permission probe rejection degrades to unknown; no voice production change in this PR.

Verification:

• npm run -w @maka/desktop test passed (1727 tests)
• npm run -w @maka/desktop build:renderer passed, with existing Vite large chunk warning
• npm run -w @maka/ui test still fails on existing chat-primitives footer-action / lineage-badge pixel merge assertions; those files are untouched here