fix(web): align onboarding and sidebar with console layouts

[sam2tom]

The introduction used oversized headings, large gaps and decorative cards that differed from the console. Its two-column layout also persisted on narrow screens, while the signed-in sidebar compressed the Core connection name beside the sign-out action.

Use the console's compact typography, spacing, borders and shadows for onboarding and account access. Stack the introduction and setup form on narrow screens, keep the language control reachable, and give the sidebar connection its own row with preferences and sign-out below.

Validation:

• Existing first-run, keyboard and recovery Playwright acceptance: 13 passed.
• Temporary browser layout checks: wide (1440), tablet (768), mobile (390) screenshots inspected; introduction and footer have no horizontal overflow; mobile account setup remains usable.
• Web TypeScript check and production build passed.
• make check ran through distribution Go tests, 108 installer tests, 12 manifest tests and console build, then stopped because PARSAR_AGENTS_API_TEST_DATABASE_URL is not configured (also checked in login shell). No complete full-gate claim.
• CSS-only change, self-reviewed. No API contract or service changes.

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[sam2tom]

Closed by product decision. Future Core Console work follows #96; this standalone change is no longer needed.