fix(chat): ID-based grouped session dropdown + UI alignment, scroll, multi-agent nav

[christineyan4]

Summary

UI polish and multi-agent fixes for the companion app chat surface.

Changes

• Chat state caching — Persist last-known thread title, model, and available models so the UI shows meaningful labels while reconnecting instead of generic placeholders
• Session dropdown ordering — Sort 'main' agent first, then alphabetical
• Assistant bubble alignment — Continuation messages now align with the first message's bubble (invisible 36px spacer in avatar column)
• User message clipping — Increased right margin (8→20px) to prevent bubbles hiding behind the scrollbar overlay
• Tool expand scroll fix — Expanding/collapsing tool call cards no longer auto-scrolls to the bottom; cards unfurl in place
• Dropdown padding — Reduced agent header top padding for symmetrical whitespace; adjusted single-group left padding
• Workspace file caching — Per-agent Dictionary cache with thread-safe locking so switching agents doesn't re-fetch from gateway
• Agent nav fix — Apply cached AgentsList on HubWindow bind so agents that arrive before the window opens are visible
• Gateway service — Request agents list on connect; extract agentId from agents.files.list response

Testing

• All 1191 existing tests pass
• Manual verification of all UI changes
• Two rounds of adversarial code review — fixed timer disposal, thread-safety, empty-key guard, and auto-scroll regression