[Improve] Keep rendered content legible across IDE themes

[zoomote]

​Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Repaired code-block fallback and toolbar colors so rendered code remains on a valid theme-derived background.
• Replaced fixed red, green, blue, gradient, and compounded-opacity treatments in command permissions, MCP tools, Marketplace states, context menus, and checkpoints with semantic VS Code theme colors.
• Added numeric WCAG checks and four-theme visual coverage for code, diffs, terminal output, command status controls, disabled MCP text, the full Welcome landing, and empty History.
• Extracted the Welcome landing presentation from provider setup so the real first-run screen can be exercised by the component-test harness.

Why this change was made

The shared-control contrast audit left high-frequency rendered content and composed screens uncovered. Several of those surfaces still used invalid CSS fallbacks or styling that assumed a dark theme.

Impact

Code output, status controls, secondary text, and first-run/history surfaces remain identifiable in light, dark, high-contrast, and high-contrast-light IDE themes. The draft intentionally does not yet include the new snapshot baselines; they will be generated by the repository's pinned visual-regression workflow before review.

Related PRs

• Zoo Code Docs theme examples

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...rc/components/chat/checkpoints/CheckpointSaved.tsx	0.00%	0 Missing and 1 partial ⚠️

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