Audit the Zed-inspired changes and make appearance customization reusable #4

Part of #2. Depends on #3 being complete.

Users should be able to reproduce and adjust the design from #3 on their own T3 Code installation without modifying source code or maintaining a fork. Combine the audit and the reusable implementation in this issue.

Scope:

• Audit the final changes from Implement a Zed-inspired design in the fork #3. Identify every adjusted color, font, spacing, shape, icon, and layout behavior, with links to the relevant code.
• Map each adjustment to existing appearance controls, theme fields, or CSS support. Identify only the missing capabilities needed to reproduce the design.
• Turn those missing capabilities into reusable appearance features. Avoid introducing a broader configuration system than the actual design requires.
• Reuse upstream theme support where it fits. Review the current state of feat: agents can restyle T3 Code with theme colors and CSS pingdotgg/t3code#35247 and discussion Let themes reach icons, shape and motion (not only colours) pingdotgg/t3code#31692 before implementing overlapping work.
• Provide a matching preset and import/export for the appearance settings required to reproduce it, plus a way to restore defaults.
• Decide which controls apply to web, desktop, and mobile, and document any unsupported differences.

Acceptance:

• The audit accounts for every appearance adjustment from Implement a Zed-inspired design in the fork #3.
• Users can reproduce the design and customize each adjustment through supported configuration, without source edits.
• Exporting and importing the configuration reproduces the supported appearance on another installation.
• Resetting restores the standard appearance.
• Verify the affected flows in a real client, preserve keyboard access, and capture visual evidence.