fix(web): compact context picker overlays

[bobleer]

Summary

• unify dispatch, branch, and workspace picker widths with one responsive 280px overlay mixin
• compact dispatch target rows and section spacing while aligning labels and secondary text to design-system typography tokens
• align branch/workspace picker control heights and add layout regression coverage

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/ChatInputWorkspaceStripLayout.test.ts src/features/dispatch/DispatchTargetPicker.test.tsx src/tools/git/components/BranchQuickSwitch.test.tsx (33 tests passed)
• pnpm run check:web
• visual source/implementation comparison at the same 280px width; compact dispatch rows resolve to 36px and the full menu to about 407px

Remote scenarios

This is a CSS/layout-only change. It was exercised in the local desktop visual path; remote workspace, remote control, peer-device, and detached-dispatch protocols and behavior are unchanged.