Refactor chat route composition

[jasonhedman]

Summary

• make /chat a prompt-only new-chat surface that creates the durable Eve session, persists metadata, and navigates once to the session route
• resume the in-flight first turn in /chat/[sessionId] without resubmitting it, with populated conversation, input, activity, and message behavior colocated under that route
• remove the activity panel from new chat and refine the populated-chat activity UI into a compact, subordinate side menu

Validation

• pnpm check — 59 test files and 250 tests, plus lint, types, formatting, Knip, and boundaries
• pnpm build — passed with documented local placeholder environment values
• Chromium computed-style verification — activity control labels resolve to 14px at weight 300
• independent implementation reviews completed with no remaining findings