feat(ui): per-project .work-files/ navigable tab

[JeanBaptisteRenard]

Summary

Adds a sidebar tab next to Plans / Memory that lists files under each project's .work-files/ directory, with content viewing (markdown rendered, JSON pretty-printed, plain text otherwise).

• public/index.html — new tab button + pane
• public/plans-memory-view.js — extends the existing pattern (loadWorkFiles / showWorkFile)
• public/app.js — tab wiring
• public/icons.js — folder icon
• public/style.css — pane styling
• main.js — list-work-files / read-work-file IPC handlers, path-validated under <projectPath>/.work-files/ to prevent directory traversal
• preload.js — exposes the IPC channels
• eslint.config.js — new globals for the test file
• test/dom-work-files-view.test.js — jsdom smoke test (~315 lines covering tree rendering, content panel, IPC mocking, path-traversal guard)

Test plan

• npm test — 53/53 pass (was 45/45 before)
• npx eslint . — 0 errors, 217 warnings (pre-existing pattern)
• Manual: launch electron, open a project with a .work-files/ dir, click the new tab, expand tree, click a markdown file, click a JSON file

Notes

• Path validation rejects any .. traversal at the IPC boundary (server-side path.resolve + prefix check)
• Watchers debounced to refresh the tree if files appear while the tab is open
• Reuses CodeMirror bundle if syntax highlighting is desired
• Original sub-agent ran into Anthropic rate limit before committing — work was recovered from the worktree and the commit + PR opened post-reset.

Closes the .work-files/ tab spec.