feat(studio): add multi-tab right inspector with CSS panel

[func25]

This is the first approach I thought of when @miguel-heygen asked about multi-tabs. There could also be a setting to turn this on or off.

What

Add a CSS tab in the right inspector and support multi-tab behavior.

• Persist right-panel tabs in URL state (tabs) while keeping tab compatibility.
• Enforce panel state guardrails:
  • max 2 open tabs
  • default tab = Design
  • single-tab inspector expands full available height
• Add authored CSS rule editing flow in Studio.
• Add focused tests for panel state and right panel rendering.

Why

• Improve editing workflow by keeping Layers/Design/CSS in one inspector area.

How

• Refactor right panel state to rightPanelTabs + rightPanelFocusTab.
• Wire state through layout/session/selection/url hooks.
• Gate inspector rendering with feature flags.
• Save authored CSS edits via file-history flow and refresh preview.

Test plan

How was this tested?

• Unit tests added/updated
• Manual testing performed
• Documentation updated (if applicable)

[func25]

@miguel-heygen I started to think this is not a good idea :), probably showing the CSS section at the bottom of Layers makes more sense.

[miguel-heygen]

@miguel-heygen I started to think this is not a good idea :), probably showing the CSS section at the bottom of Layers makes more sense.

I'll have to think in the best UX today