Persist context token count: return from API, restore on session load ...

- GET /sessions/{id} now returns context_token_count and max_context_tokens
  (max pulled from settings.ollama_num_ctx)
- loadSession() in chat store sets contextTokens/maxContextTokens from the
  response so ContextBar shows the last known fill level immediately on load,
  not only after the first new message
- Restore v-if guard on ContextBar (hides for brand-new sessions with 0 tokens)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>