feat(chat): one-click "Revert changes" button for agent modifications (closes #85)

[haoyu-haoyu]

Summary

Adds a "Revert changes" button to each finished Coding Agent turn so users can roll back that turn's file modifications from the chat UI instead of context-switching to the terminal for git restore .. Closes #85.

What it does

• Scans the messages in the finished agent turn for Edit / Write / ApplyPatch / MultiEdit / FileChanges tool calls
• Extracts every file path those tools actually modified (handles both object and stringified-JSON toolInput, multi-file file_paths arrays, Codex's kind: path newline format, and object-map changes from patch-apply results)
• Click → inline confirmation with an explicit warning → POST /api/git/revert-agent-changes
• Backend reverts each file independently:
  • untracked regular files → unlink (directories and out-of-repo symlinks are refused)
  • tracked worktree+staged modifications → git restore --source=HEAD --staged --worktree (handles MM / AM / AD / MD / DM)
  • staged adds → git reset HEAD + unlink
  • rename / copy pairs → skipped (refuses to leave half-renamed state)
  • per-file error reporting so one bad entry doesn't abort the rest

Safety

• Only touches files this specific agent turn modified — the main ask from the issue. Any manual edits to unrelated files are preserved.
• Path traversal blocked via the existing safePath helper.
• Untracked-symlink deletion re-validates realpath() against the project root before unlink.
• git status --porcelain -z prevents filenames containing " -> " from confusing rename parsing.
• Confirmation dialog explicitly warns: "Any manual edits to these files will also be lost" — honest about git restore semantics since we restore to HEAD, not a pre-agent snapshot. Per-turn exact-snapshot revert (with conflict detection for files the user also touched) would need pre-agent content capture and feels like a separate PR.

Accessibility

• role="group" on the inline confirmation with aria-label, focus moves to the destructive action on open, Escape cancels and restores focus to the trigger
• role="status" + aria-live on reverting / done / error states
• Decorative SVGs marked aria-hidden

Test plan

All 130 Vitest tests pass (incl. 8 new backend integration tests against a real temp git repo, 1 in-process Express route test, and 12 new frontend unit tests). tsc --noEmit clean.

• npm test -- --run (130 passed, 18 test files)
• npm run typecheck (clean)
• Backend: untracked file delete, tracked file restore, staged add, mixed MM state, rename skip, rename-with-modification skip, symlink safety (external file preserved), directory rejection, path traversal blocked, filename containing " -> "
• Frontend: object toolInput, stringified-JSON toolInput, multi-file file_paths, Codex FileChanges string format, object-map changes, array-of-strings changes, malformed JSON, subagent children, tool-error skipping, no-result skipping
• Route: 400 on missing project, 400 on non-array files, empty list, happy path, partial success reporting
• Manual smoke test: run agent, modify multiple files, click Revert, confirm only those files roll back

i18n

Added revertAgent block to en, zh-CN, ko chat locales with i18next pluralization (_one / _other).

Follow-ups (not in this PR)

• Per-turn exact-snapshot revert with conflict detection (requires capturing pre-agent content)
• Rename handling: currently skipped; could surface a custom UI prompt
• Korean locale has some unrelated pre-existing missing keys (providerInfo.nano, readyPrompt.nano) — left for a separate i18n-sync PR