fix(ui): align menu spacing and restore intended text contrast

[kev1n77]

Summary

• Remove extra menu row spacing and focus padding while keeping keyboard focus indicators inside scrollable items.
• Align menu shadows, hover/pressed backgrounds, caption typography, and navigation heading height with the design.
• Restore the intended menu label contrast in the built-in light Appearance.
• Add a scrolling option to the Design Lab nested-menu example.

Type and Areas

Type: Bug fix / UI/UX

Areas: Design system, Web UI, Appearance themes, Design Lab

Motivation / Impact

Product menu labels appeared too faint because the built-in light Appearance mapped neutral action text to secondary text at 60% opacity instead of the public theme's intended 80%.

Menu rows now remain contiguous at 30px, without additional list or heading-to-item gaps. Separators retain their own 8px vertical spacing. Menu and navigation captions use a dedicated color token, avoiding compounded opacity.

Verification

Passed during implementation:

• pnpm run design-system:check
• pnpm run check:web
• 27 focused regression tests across three Web UI test files.

Regression coverage includes Appearance output, legacy overrides, and component layout contracts.

Full visual verification across hover, pressed, checked, disabled, keyboard focus, scrolling, themes, and density settings remains pending. Relevant entry points are the chat input “+” menu and Design Lab's nested-menu, Menu, ActionItem, and NavigationPanel examples.

Reviewer Notes

• Existing imported Appearance overrides remain supported. When the new caption token is absent, an explicit legacy muted color is preserved.
• Explicit imported action colors retain precedence.
• No backend protocol or persisted data shape changes.
• Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not tested end to end.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.