feat(studio): add desktop floating control rail

[tlgimenes]

Summary

• move desktop organization and agent switchers into the sidebar and let panel cards use the full viewport height
• add an icon-only floating rail ordered as Git actions, Chat/Library/conditional Tasks, then agent actions
• group desktop preview controls beneath Preview while preserving mobile controls and window-controls-overlay safety
• remove desktop topbar and panel-tab components made obsolete by the new rail

Testing

• bun run fmt
• bun run --cwd apps/mesh check
• bun run lint (0 errors)
• 135 focused tests passed
• bun run knip
• bun run --cwd apps/mesh build:client
• git diff --check

Screenshots

• Not captured in this environment.

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Summary by cubic

Adds a desktop floating control rail that replaces the top header controls and frees panels to use full height. Moves the org/agent switchers into the sidebar and integrates preview controls into the rail on desktop while keeping mobile controls unchanged.

• New Features

  • Floating rail with icon controls for Git actions, Review changes, Chat, Library, Tasks, and agent tabs; includes a preview controls portal on desktop.
  • Sidebar header shows an org/agent breadcrumb (compact when collapsed) with pending-invite indicator.
  • Window-controls-overlay safe areas on desktop with a drag region and safe padding for content.

• Refactors

  • Removed the desktop topbar and main-panel tab bar; actions and tabs are now on the floating rail.
  • Preview controls: desktop via the rail; mobile keeps the bottom floating controls.
  • GitHub header actions support a rail variant with semantic icons and tooltips.
  • Breadcrumbs render in the sidebar header and mobile toolbar; support compact agent avatar trigger.
  • Content cards (Library, Task Board, workspace) use desktop WCO-safe wrappers for full-height layouts.
  • Added a dev/live mode switch inside the agent picker; removed the old dev agent toggle component.

^{Written for commit bfbc120. Summary will update on new commits.}