File field: "Select Existing File" functionality

[oidacra]

Parent Issue

#29691

Task

Implement the "Select Existing File" feature, which opens a popup for users to select a file already existing in dotCMS.

Proposed Objective

User Experience

Proposed Priority

Same as Parent Issue

Acceptance Criteria

1. A "Select Existing File" option is available in the File field
2. Clicking the option opens a popup similar to other content selection popups
3. The popup allows browsing and searching for existing files in dotCMS
4. Selected file is properly linked to the File field upon confirmation
5. The popup is responsive and works well on different screen sizes

Assumptions & Initiation Needs

• API for retrieving existing files in dotCMS is available
• Design specifications for the file selection popup are provided

### Tasks
- [ ] https://github.com/dotCMS/core/issues/30214
- [ ] https://github.com/dotCMS/core/issues/30215
- [ ] https://github.com/dotCMS/core/issues/30216
- [ ] https://github.com/dotCMS/core/issues/30217