fix(app): respect custom opencode project icons

[angristan]

Issue for this PR

Fixes #27882
Fixes #26888
Fixes #24744
Fixes #24197

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The opencode project has a default hard-coded favicon. That default was returned before checking user-provided icon overrides, so an uploaded project icon could be saved but still not display for that project.

This changes the avatar-source precedence to prefer uploaded overrides first, then color avatars, then the opencode favicon fallback. It also adds regression coverage for that precedence.

How did you verify your code works?

Verification

• bun test src/pages/layout/helpers.test.ts
• bun typecheck

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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