R2: Reorganize Desktop shell for IM command center

[DeliciousBuding]

Summary

• introduces the Desktop IM-first shell structure with Global Rail and workspace split
• adds activeRailView state coverage
• fixes R2 visual QA blockers: useSidebarResize now uses useShallow, the mobile top bar hides desktop-only menu/status chrome under 767px, and the UI capture script now honors --theme light
• cleans R2-local App.tsx lint errors from unused imports/state and render-time ref writes
• syncs with dev/delicious233 commit 98fda1ab, which fixes the Desktop/validate CI baseline for this PR

Validation

• corepack.cmd pnpm exec vitest run src/__tests__/uiStore.test.ts (4/4)
• corepack.cmd pnpm typecheck
• corepack.cmd pnpm test:ci in the R2 worktree: 420 + 325 + 24 + 335 tests passed
• corepack.cmd pnpm exec eslint src/App.tsx --max-warnings 999 (0 errors; 4 existing warnings)
• Playwright console check at 1440x920 and 390x844: no ErrorBoundary / pageerror; only Vite/React dev info
• light shell screenshots regenerated locally: desktop-1440x920-light-home.png, desktop-1280x800-light-home.png, desktop-390x844-light-home-fixed.png
• git diff --check origin/dev/delicious233..HEAD
• Claude sonnet read-only reviews: PASS, no actionable findings for the R2 visual QA diff or Desktop CI baseline diff

CI status

The direct R2 blockers in frontend-desktop and validate have been fixed on dev/delicious233 and merged into this branch. The new PR run has validate green and frontend-desktop is expected to use the fixed baseline.

Repo-wide checks may still be red outside the R2 shell diff:

• Hub Docker build
• Web lint/build/test baseline
• Mobile typecheck/build/test baseline
• Go/E2E baseline checks, depending on the current Actions run

Merge notes

Code is ready for review as the R2 shell foundation, but do not merge while required GitHub checks remain red. Final competition visuals still need R3/R4/R7 on top: real transcript blocks, right-side evidence column, and TeamRun artifact/run proof.