feat(playground): add MCP-first landing page

[devallibus]

Summary

• show an MCP-first onboarding card on /playground when no session query param is present
• keep the existing editor flow for valid sessions and add a recovery state for invalid session links
• include the bundled TSL error-reporting cleanup from PR fix(playground): report structured TSL errors #75 review feedback

Testing

• bun run test:web
• bun run build:web

Closes #69

[devallibus]

Review: Approve (self-PR — submitted as comment)

Solid PR. Clean MCP-first landing page with good state management and a well-factored TSL error-reporting refactor. CI passes. No blockers.

Suggestions (non-blocking)

1. suggestion: Use <Switch>/<Match> instead of nested <Show> — playground.tsx:90-146
   The two mutually-exclusive <Show when={X}> / <Show when={!X}> with a ternary in the fallback obscures the state machine. <Switch>/<Match> would make the four states (landing / loading / error / session) explicit.

2. suggestion: Add role="alert" to error containers — PlaygroundLanding.tsx:153, playground.tsx:117
   Error messages appear dynamically but have no ARIA live region — screen readers won't announce them.

3. suggestion: Use semantic <ol> for the 3-step instructions — PlaygroundLanding.tsx:54-96
   Steps are visually numbered but use flat <div>s. <ol>/<li> gives screen readers "item X of Y" context.

4. suggestion: Rate-limit createManualSession — playground.tsx:17-23
   This POST creates a new SQLite row with no rate limiting. The reviews system already has per-IP limiting — same pattern should apply here.

5. nitpick: Remove dead setLoading(false) — playground.tsx:77
loading is already false when this path runs (no session param on landing).

6. nitpick: Use <Show when={props.error}> instead of ternary — PlaygroundLanding.tsx:153
   Consistency with the rest of the codebase.

Test coverage notes

• tsl-error-reporting.ts: 3 untested branches in getMessage (string input, non-Error/non-string, empty message) — quick win in existing test file.
• Route state machine: 8 transitions at 0% coverage — best addressed with E2E tests (future work).