Agent tab strip & transcript usability improvements

[DevMando]

Summary

A round of usability polish for two everyday surfaces in the desktop app: the agent tab strip at the top and the chat transcript. No changes to model behavior, data handling, or the core chat flow — this is about making the app easier to navigate and read, especially when a user has many agents open or the assistant produces large outputs.

Agent tab strip

• Right-sized tabs. Tabs are a comfortable width when there's room and only shrink as more agents are opened; past that point the strip scrolls (mouse wheel, touch, or scrollbar). Previously tabs were always small.
• No more overlapping scrollbar. The scrollbar now sits in its own reserved lane instead of drawing on top of the tabs.
• Clearer structure. Added subtle dividers separating the brand, the tabs, and the add button, with the header aligned to the tabs.
• Always-reachable "New agent" button. The add-agent action is now a permanent, labeled button pinned to the far right, so it never scrolls out of reach when many agents are open.
• Follows the active agent. Creating or selecting an agent scrolls that tab fully into view, so the selected agent is never hidden off-screen.
• Wording. The per-tab menu item now reads "Close agent" (was "Close tab"), matching the app's "agent" language.

Transcript

• Collapsible large panels. Long diffs and command output collapse to a short preview with an Expand/Collapse control, so a big file write no longer fills the screen and forces endless scrolling.
• Quieter web results. Web search/fetch result previews are hidden behind an Expand toggle to cut down on noise.
• Theme-consistent diffs. The approval/plan diff preview now uses the active theme's colors instead of a hardcoded blue (it previously looked out of place on non-default themes).

Risk / scope

• Low risk, presentation-layer only. Changes are limited to layout and rendering in the desktop UI; no new dependencies.
• Builds clean (0 warnings, 0 errors).
• Existing behaviors (selecting/closing agents, chat background, window opacity, approvals) are unchanged aside from the layout/wording noted above.

Verification

• Manually exercised: opening many agents (sizing, scrolling, chevron-free scrollbar, permanent add button), creating/selecting agents (auto-scroll into view), collapsing/expanding large diffs, and toggling web-result previews.