feat: guide empty first-run spaces

[lilyshen0722]

Closes #687

Summary

• show a compact Join Commonly HQ offer only when HQ is configured and the current user is not already a member
• teach users how visible pod conversation works in the regular empty state while preserving the bot-to-bot branch
• add agent-DM starter prompts that fill and focus the composer without sending, then disappear once the user types or messages exist
• add token-based responsive styles and layout invariants for the HQ card and wrapping prompt chips

Verification

• full frontend tests: 54 suites, 241 tests passed
• focused onboarding/layout tests after rebase: 4 suites, 22 tests passed
• frontend production build passed
• changed-file ESLint: 0 errors (repo JSX-extension warnings only)
• full frontend ESLint reaches the existing unrelated PersonalityBuilder.tsx react/no-unescaped-entities error; this branch introduces no lint errors
• real-browser checks passed at 1280x900 and 390x844: no horizontal overflow, the mobile drawer card remains fully visible, chips wrap within the viewport, click fills/focuses the composer, and no message POST occurs

Scope notes

No backend, route, or API changes. The Community redirect from #699 remains the single member/non-member destination.