Improve searchbox UX: select text on Ctrl+F and add Enter / Shift+Enter navigation #1613

Is your feature request related to a problem? Please describe.
Searching for text across QualCoder currently has a few minor UX inconsistencies:

1. No automatic text selection on Ctrl+F: In several screens (such as Text Coding, Journals, and Coding Reports), pressing Ctrl+F focuses the search box without selecting the existing text. The user has to manually press Ctrl+A or Backspace before pasting or typing a replacement query.
2. Missing window-wide shortcut in some views: In Audio/Video coding (code_av), Media viewing (view_av), and summary reports, Ctrl+F is not bound at the window level to jump to the search box.
3. Inconsistent search box keyboard navigation:
   • In Text Coding (code_text), pressing Shift+Enter in lineEdit_search triggers Qt's default returnPressed signal, which moves to the next result rather than the previous one.
   • In Journals and Audio/Video coding, pressing Enter or Shift+Enter inside the search box does not navigate results at all (requiring mouse clicks on the next/previous arrow buttons).

Describe the solution you'd like
A unified and consistent search keyboard interaction across the application:

1. Ctrl+F selection: Pressing Ctrl+F anywhere in a dialog—or while already inside the search box—should focus the search box and call selectAll(), so that typing or pasting immediately replaces any previous search term.
2. Enter for Next: Pressing Enter / Return inside the search box advances to the next search match.
3. Shift+Enter for Previous: Pressing Shift+Enter / Shift+Return inside the search box navigates to the previous search match in screens supporting bidirectional search (code_text, code_pdf, code_av, view_av, journals, edit_textfile), and safely no-ops in forward-only report screens.
4. Reusable helper: Centralize this behavior in src/qualcoder/helpers.py (e.g. setup_search_shortcuts()) to keep all screens consistent without duplicating event filter logic.

Describe alternatives you've considered

• Using F3 / Shift+F3 for Next/Previous: Standard in some desktop text viewers, but in QualCoder coding views F3 is already globally assigned to "Code / Cat Memo" on the code tree widget. Keeping navigation to Enter and Shift+Enter inside the search box avoids keybinding conflicts.
• Implementing per-dialog event filters: Leads to duplicated code across ~10 dialog classes and risks subtle differences in modifier handling.

Additional context
I have already implemented and tested this change locally across all coding views, journals, text editing, and report dialogs, along with a dedicated unit test suite (tests/test_search_shortcuts.py). I would be happy to submit a Pull Request if this feature is welcome!