Fix desktop visible error states / 修复桌面可见错误状态

[GTC2080]

Summary

• Split Settings initial loading from failures so rejected startup settings reads show a safe error message and retry path instead of an endless loading state.
• Preserve Workspace Changes git metadata in frontend state and show a non-blocking warning when git status is unavailable or failed, while still rendering session files.
• Surface non-image paste/drop attachment failures through the existing toast path without clearing composer content or exposing local paths in user-visible text.

Verification

• git diff --check
• (cd desktop/frontend && corepack pnpm test)
• (cd desktop/frontend && corepack pnpm build)
• (cd desktop && go test ./...)
• go test ./...

Cache impact

Cache-impact: none - desktop frontend visible error/empty-state handling only; no provider-visible prompt, tool schema, request serialization, memory prefix, or cache-sensitive behavior changed.
Cache-guard: (cd desktop/frontend && corepack pnpm test) covers the new Settings, Workspace Changes, and Composer regressions.
System-prompt-review: N/A

[SivanCola]

Thanks, I reviewed this against the latest main-v2 and the implementation looks directionally right: the Settings panel now exits the endless loading state on startup read failure, Workspace Changes keeps rendering session files while surfacing git-status failures, and pasted/dropped non-image attachment failures now use the existing toast path without exposing exception text.

I cannot approve it in the current state because the PR head is now conflicting with main-v2. Please rebase onto the latest main-v2 and resolve these conflicts before merge:

• desktop/frontend/package.json
• desktop/frontend/src/__tests__/composer-goal-toggle.test.tsx
• desktop/frontend/src/components/WorkspacePanel.tsx

When resolving WorkspacePanel.tsx, please preserve the newer code-preview/body class handling from main-v2 and then reapply the WorkspaceChangesView state shape plus git-unavailable warning. When resolving composer-goal-toggle.test.tsx, please keep the newer composer draft/session coverage from main-v2 and add this PR's attachment-failure assertions on top.

Related PR scan: this overlaps with Workspace Changes work in #4843, #4987, and #5245. I think this PR can still merge independently after rebase because it is narrowly about visible error states, but the final rebase should re-check the combined Workspace Changes UI.

Security/cache review: pass. I did not see new dependencies, network calls, command execution, privilege expansion, provider-visible prompt changes, tool-schema changes, provider request serialization changes, compaction changes, or reasoning upload changes.

Verified on the PR head:

• git diff --check origin/main-v2...HEAD passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/workspace-changes-errors.test.tsx passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/settings-refresh-snapshot.test.tsx passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/composer-goal-toggle.test.tsx passed
• GitHub checks are currently green on the old head, including desktop, platform tests, CodeQL, and cache-impact

Once the conflicts are resolved and CI stays green on the rebased head, I will consider merging it into main-v2.

[SivanCola]

Pushed e3a7ad9a7 to resolve the main-v2 conflicts on this PR.

What changed:

• Merged the latest main-v2 into the PR head.
• Kept the newer WorkspacePanel split/code-preview behavior from main-v2 and reapplied the Workspace Changes git-unavailable warning path.
• Kept the newer composer drop-navigation regression and retained this PR's pasted/dropped attachment failure coverage.
• Included workspace-changes-errors.test.tsx in the updated frontend test script list.

Verified after the merge:

• git diff --check passed
• corepack pnpm --dir desktop/frontend exec tsc --noEmit passed
• corepack pnpm --dir desktop/frontend check:css passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/workspace-changes-errors.test.tsx passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/settings-refresh-snapshot.test.tsx passed
• corepack pnpm --dir desktop/frontend exec tsx src/__tests__/composer-goal-toggle.test.tsx passed

Note: corepack pnpm --dir desktop/frontend exec tsc --noEmit -p tsconfig.test.json still reports pre-existing test typing issues in unrelated test files (composer-profile.test.ts and use-controller-meta.test.ts).

GitHub now reports the PR as mergeable; required checks are still running on the new head.