fix(ui): align page headers and action cards with the design system

[kev1n77]

Summary

• Give welcome and settings page headers distinct typography, using the shared PageHeader in the welcome screen.
• Remove redundant settings-header typography and spacing overrides.
• Align action cards with the design: a translucent 3% background in light mode, 12px padding, 15px semibold titles, and clearer 13px descriptions.
• Use outlines instead of borders to avoid inflating card geometry.
• Update Design Lab previews and preserve legacy Appearance background overrides.

Type and Areas

Type: Bug fix / UI/UX

Areas: Web UI, shared UI components, design tokens, themes, Appearance compatibility, Design Lab.

Motivation / Impact

Welcome and settings headers previously used inconsistent typography, while action cards had mismatched spacing and overly muted descriptions.

This change gives page introductions and entry cards consistent visual hierarchy across product screens and Lab previews. Long content remains compatible with trailing actions, and existing custom Appearance backgrounds retain their precedence.

Verification

Recorded validation for this patch:

• pnpm run design-system:check — passed, including component checks and the Lab production build.
• pnpm run check:web — passed, including theme audits and TypeScript checks.
• Focused Appearance regression tests — 2 files, 19 tests passed, covering legacy payload round trips and explicit new-token precedence.
• Native Windows desktop inspection covered settings headers and global-search entry cards.

Checks were not rerun while drafting this description. Other themes, narrow layouts, and remote interaction scenarios remain pending visual verification.

Reviewer Notes

• Welcome headers use the display variant; settings headers use md.
• Action-card sizes remain distinct: sm and md retain their respective minimum heights.
• The new card-background token preserves legacy Appearance overrides in both root and chrome scopes.
• No backend protocols or persisted data shapes change.

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.