Polish app chrome and composer interactions

[jsgrrchg]

Summary

Adds a round of interaction polish across the app chrome, sidebar, vault switcher, and AI composer so the UI feels more tactile and stable during common power-user flows.

What Changed

• Added hover, active, focus, and open-state feedback to sidebar tabs and agent sidebar toolbar buttons.
• Made composer controls feel more physical, including tactile press states for send/stop buttons and agent dropdown controls.
• Stabilized the composer footer during stop/queue transitions so controls no longer flash or shift when stopping a streaming response.
• Kept the send action anchored while the stop button fades in and out, reducing layout movement during AI runs.
• Redesigned the vault switcher trigger as a compact inset card with clearer pressed/open states and improved menu item feedback.
• Updated the composer expanded-state icon so it correctly communicates collapse behavior.

Why

This branch focuses on polish and hardening. A few controls were technically functional but felt flat, and the composer action row could visually jump during stop/queued-send states. The composer footer needed a small bounded refactor so the UI preserves layout while state changes crossfade cleanly.

Testing

• npm run lint
• npm run build

Manual QA

Recommended checks:

• Sidebar tab hover/press/active states.
• Vault switcher open, search, select, and context menu behavior.
• Composer send/stop behavior while streaming.
• Agent control dropdown open/close/search behavior.
• Expanded/collapsed composer icon affordance.