feat(v2): strengthen create and first-run onboarding

[lilyshen0722]

Summary

• promote the existing first-five-minutes guide from the chat transcript to a shell-level modal without changing its visibility, polling, engagement, or dismissal model
• add modal accessibility guarantees: dialog semantics, initial focus, focus restore, Escape/backdrop dismissal, focus trap, and mobile internal scrolling
• keep the existing create flow intact while adding one-line explanations for Team vs Private policy in both locales
• give the already-shipped just-created Pod starter panel a bounded visual-weight bump only

Verification

• cd frontend && npm test -- --watchAll=false --runInBand — 64 suites, 293 tests
• cd frontend && npm run build — pass
• focused first-run / create / starter / i18n / layout suites — 45 tests
• changed-file ESLint — 0 errors
• real Chromium: EN + zh-CN at 1280x900 and 390x844; modal and create-policy helpers stay within viewport with zero horizontal overflow

Guarantee coverage

• modal role + aria-modal, focus-on-open and prior-focus restore
• Escape and backdrop both persist the existing dismissed flag
• dismissed/connected users do not see the modal
• chat empty state returns after dismissal; starter invite is not minted while first-run owns the shell
• Team and Private helper copy renders from both locale catalogs

Review gate

The two new zh-CN.json policy helpers are draft UI copy following the locked glossary. Merge is blocked on Sam native-speaker review of the zh-CN diff.

[lilyshen0722]

Temporarily closing and reopening to refresh GitHub's stale PR-head metadata after restoring this branch to the approved D+E-only commit d873370; F+G now lives separately in #735.

[lilyshen0722]

Superseded by #736, which points at the exact approved D+E-only tree . GitHub detached this PR's internal pull ref after the branch restore and kept (the now-separated F+G work) despite the real branch moving; merging this PR would therefore violate its reviewed scope. F+G lives only in #735, now stacked on #736.

[lilyshen0722]

Correction to the shell-stripped hashes above: replacement #736 points at exact approved D+E-only tree d873370. This PR internal pull ref remained at 5d59e9e, which contains the now-separated F+G work. #735 is the sole F+G PR and is stacked on #736.