[Desktop & Web] @ mention popup needs style/interaction improvements and shows incomplete data

### Self Checks
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### Syncless version
Latest

### Platform / Device
- macOS Desktop App
- Web (Browser)

### OS version
macOS

### Steps to reproduce
1. In the message input, type `@` to trigger the mention popup.
2. Observe the popup list (devices, teammates, skills).
3. Try to find a specific target without knowing its exact keyword.

### ✔️ Expected Behavior
- The popup should display a complete list of all available targets (devices, teammates, skills, etc.).
- The style and interaction should be clear and easy to navigate (e.g., searchable, scrollable, categorized).
- Users should be able to browse all options even without knowing the exact keyword.

### ❌ Actual Behavior
- The popup shows incomplete data — not all available items are listed.
- Without knowing the target keyword, users cannot find what they are looking for, resulting in poor discoverability and usability.
- The popup style and interaction design need improvement for clarity and efficiency.

### Additional context
N/A

Start by reproducing the mention popup from the message input on the macOS Desktop App and Web, then inspect how devices, teammates, and skills are presented. Done means the popup exposes all available targets and provides clear, usable browsing or search interactions without requiring an exact keyword.

Written by the indexing model from the issue text.