fix(web): refine workspace and target pickers

[bobleer]

Summary

• unify picker width, compact row rhythm, button height, and typography across workspace, branch, and execution-target menus
• show repository paths and assistant roles as 50%-opacity secondary lines without exposing assistant workspace paths
• keep the selected branch readable in light and dark themes without disabling the current-branch row
• remove the redundant execution-target menu preamble so the picker starts at the local target section

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/ChatInputWorkspaceStrip.test.tsx src/flow_chat/components/ChatInputWorkspaceStripLayout.test.ts src/features/dispatch/DispatchTargetPicker.test.tsx src/tools/git/components/BranchQuickSwitch.test.tsx (64 passed)
• pnpm run check:web
• pnpm run i18n:audit

Remote scenarios

• Local Desktop/Web UI picker behavior and theme contracts were exercised.
• Remote-workspace paths are rendered from rootPath unchanged; no path joining or transport behavior changed.
• Remote control, Peer Device Mode, and Detached Dispatch behavior is unchanged and was not exercised.