Perform text searches within individual entries

### Checks

- [x] I have read the Wiki, searched the open issues, and still think this is a new feature.

### Explain the problem clearly and succinctly:

Currently, KeePassDX offers excellent global search functionality across the entire database. However, when working with entries that contain extensive notes, multiple custom fields, or lengthy descriptions, it would be incredibly helpful to have a search function that works within the opened entry itself.

_User request by email_

### Describe the solution you'd like:

This feature would be particularly useful for:
- Quickly locating specific information within detailed notes
- Finding particular keywords in entries with multiple custom fields
- Navigating through lengthy entries more efficiently
- Reducing the need to scroll through extensive text content

### Describe alternatives you've considered:

_No response_

### Additional context:

_No response_

No implementation files, tests, or entry points are named. Start by locating the opened-entry screen and its existing notes and custom-field views; determine which entry content is searchable and how matches should be navigated, then verify searching within lengthy notes and multiple custom fields.

Written by the indexing model from the issue text.