Sandbox project picker with main project name

[elias-ba]

Description

When you're inside a sandbox, the project picker now shows both the parent project and sandbox name (e.g. ethiopia-bdr:feb-red-team) instead of just the sandbox name. The button gets the sandbox accent color and a beaker icon so you can tell at a glance that you're in a sandbox.

The sidebar and navbar no longer change color when you're in a sandbox. The accent color only lives on the picker button itself.

Inside the picker overlay, sandboxes show up as indented children under their parent project, with support for arbitrary nesting depth. Searching works across the full hierarchy path.

I also consolidated things: the picker button is now a single React component (ProjectPickerButton) shared between the standard HEEx pages and the collaborative editor, and the picker overlay is a single global_project_picker component shared across all layouts. The old ProjectTheme module and theme preview code were no longer needed so I removed them.

Closes #4510

Validation steps

1. Hit Cmd+P to open the project picker and check that sandboxes appear nested under their parent
2. Pick a sandbox, then pick the parent project, check the button updates correctly
3. Check the sidebar stays in default colors when inside a sandbox
4. Try it from different pages: workflows, settings, history, channels
5. Open a workflow in the collaborative editor and check the picker button there too
6. If you have nested sandboxes (sandbox of a sandbox), check the tree indentation and the truncated display name with tooltip

AI Usage

• I have used Claude Code
• I have used another model
• I have not used AI

Pre-submission checklist

• I have performed an AI review of my code (we recommend using /review with Claude Code)
• I have implemented and tested all related authorization policies.
• I have updated the changelog.
• I have ticked a box in "AI usage" in this PR

[github-actions]

Security Review

⚠️ Automated security review did not complete.

Claude hit the max-turns limit or encountered an error before posting findings.
A manual review of S0 (project-scoped data access), S1 (authorization policies),
and S2 (audit trail coverage) is recommended for this PR.

See the workflow run for details.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (2f1a43e) to head (8800cca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4612   +/-   ##
=======================================
  Coverage   89.61%   89.62%           
=======================================
  Files         444      443    -1     
  Lines       21505    21494   -11     
=======================================
- Hits        19272    19264    -8     
+ Misses       2233     2230    -3     

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[theroinaochieng]

Hey Elias, this looks really good. Great job.