feat(v2): clarify selected tabs and reopen the guide

[lilyshen0722]

Summary

• make the shared filter-segment selected state unmistakable with solid cobalt, white text, and bold weight; both the main pod filters and Community sub-tabs inherit the one rule
• remove the duplicate weak pod-filter override so it cannot silently win the cascade again
• add a persistent localized Guide / 指南 action beside Feedback and Sign out in the nav-rail footer
• reopen the existing first-run modal through a tiny window event, clearing dismissal and setting the existing started/engaged latch so connected users reopen correctly too
• leave the composer's already-strong accent border and focus ring unchanged

Stacked PR

This companion is intentionally based on #734 head d8733704 because it touches V2FirstRunHero.tsx and v2.css. The PR targets codex/732-create-first-run-ux so review shows only F/G. After #734 merges, rebase this branch onto main and retarget the PR.

Root cause

The sidebar's selected state was both visually weak and split across a shared segment class plus a duplicate pod-specific override. The shared rule now owns the treatment for both rows. Its selector includes the v2 root/button scope so the global button reset cannot erase the background, border, or text color.

Verification

• cd frontend && npm test -- --watchAll=false --runInBand — 64 suites, 298 tests
• cd frontend && npm run build — pass
• focused F/G suites — 4 suites, 44 tests
• changed-file ESLint — 0 errors
• git diff --check — pass
• real Chromium: EN + zh-CN at 1280×900 and 390×844
  • both active tab tiers compute to rgb(47, 111, 235) background/border, white text, weight 700
  • Guide / 指南 reopens for a connected user, focuses the dialog, sets started=1, and Escape restores focus to the footer action
  • composer retains the existing cobalt border + 3px focus ring
  • zero horizontal overflow and zero page errors in all four cells

Guarantee coverage

• shared selected-state invariant asserts --v2-accent, rejects --v2-accent-soft, and pins white token-backed text + weight 700
• connected and unconnected dismissed-user tests both reopen, assert both storage transitions, and dismiss normally
• rail test proves the localized action emits the one shared reopen event
• shell integration proves reopening suppresses the chat empty state and resumes the existing connection probe

Review gate

The only new zh-CN copy is firstRun.reopen: "指南". Merge remains blocked on Sam's native-speaker review.