feat(station-viewer): Add chat prototype

[r3nya]

Summary

Adds a feature-flagged Chat route to station-viewer.

The chat UI is hidden by default behind features.json (chat: false). When enabled, the app shows a /chat route and navigation link with a Telegram-like layout: sessions on the left and the active conversation on the right.

What changed

• Added FEATURE_FLAGS loading from features.json and guarded the Chat route/navigation entry.
• Added a Chat page split into focused src/chat/* modules for controller logic, formatting, message grouping, scroll behavior, session state reduction, and presentational components.
• Added a NormFS-backed chat store:
  • session events: session.create, session.update, session.delete
  • message events: message.create
  • deletion is modeled as an append-only session tombstone/state-machine event, not as physical NormFS deletion
• Added chat UX basics:
  • session search/filter
  • new chat, refresh, and delete session actions
  • text-only composer
  • Enter inserts newline, Ctrl+Enter / Cmd+Enter sends
  • optimistic message sending with pending, saved, and failed states
  • retry for failed message saves
  • empty/offline states
  • date separators and compact message timestamps
• Added large-history handling:
  • initial message load reads the latest page from the NormFS tail
  • scrolling near the top loads older messages by absolute NormFS entry index
  • prepending older messages preserves the reader's scroll position
  • auto-scroll only happens when the user is already near the bottom
• Added responsive behavior:
  • desktop shows sessions and conversation side by side
  • mobile switches between session list and conversation screens instead of squeezing both columns

Notes

• The feature flag is disabled in the PR by default. Local checkouts can flip software/station/clients/station-viewer/src/features.json to {"chat": true} for manual testing.
• NormFS does not expose physical delete here; session delete is represented as a queued session.delete event and reduced in client state.

Validation

• npm run type-check
• npm run lint (passes with existing warnings outside the chat code)
• npx vite build (passes with the existing large chunk warning)
• Browser smoke checked /chat desktop and mobile layouts locally.