Add visual indicators for user-added models

[Copilot]

Description:

User-added models (downloaded but not in models.json) now display a "Custom" badge in the model selector. The capabilities tooltip warns users that these models may have unspecified capabilities.

Related Issue:

Improve UX for user-added models

Changes Made:

• ModelSelector.vue: Added "Custom" badge next to models where isPredefined === false

  • Appears in dropdown list and selected state
  • Styling: bg-primary/20 text-primary with 10px font

• ModelCapabilities.vue: Added warning in info tooltip for user-added models

  • Shows: "⚠️ User-added model - capabilities may not be fully specified"
  • Only displays when isPredefined === false

Capabilities tracking already works - predefined models have full metadata from models.json, user-added models show "Standard" when capabilities are undefined.

Testing Done:

• Code review: No issues
• CodeQL security scan: Clean
• TypeScript compilation: Success

Manual testing required:

• Verify "Custom" badge appears for models not in models.json
• Verify warning displays in capabilities tooltip
• Verify no badge/warning for predefined models
• Test visual consistency across light/dark themes

Screenshots:

Screenshots require running the Electron app. See VISUAL_CHANGES.md for ASCII mockups of the UI changes.

Checklist:

• I have tested the changes locally.
• I have self-reviewed the code changes.
• I have updated the documentation, if necessary.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve UX for user-added models</issue_title>
<issue_description>* indicate which models are not "default", i.e. added by the user

• figure out how to track capabilities of user-added models</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve UX for user-added models #116

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