feat: refactor settings UI with category navigation and add Delimiters page

Closes #640

## Changes

### UI Refactoring
- Replace monolithic settings dialog with category-based navigation
- Add QListWidget (lw_categories) for category selection on the left
- Add QStackedWidget (sw_categories) for content pages on the right
- Organize settings into 5 distinct pages:
  - Flags (14 checkboxes)
  - Quality (3 combo boxes)
  - Numbers (2 spin boxes)
  - Paths & Formats (7 line edits + 2 browse buttons)
  - Delimiters (4 line edits) — NEW

### New Feature: Delimiters Category
- Add customizable separators for artists in metadata and filenames:
  - metadata_delimiter_artist
  - metadata_delimiter_album_artist
  - filename_delimiter_artist
  - filename_delimiter_album_artist
- Auto-integration with DialogPreferences via parameters_line_edit
- Line edits with max width 100px for compact display

### Styling
- Apply dark theme to settings dialog:
  - Category list: #2b2b2b background, #3d5a80 selection, #3a3a3a hover
  - Settings area: #333333 background, styled GroupBoxes (#3a3a3a)
  - Light text (#e0e0e0) for better contrast
- Add spacing property (6px) to category list for better readability
- Style QListWidget items with padding (4px 3px) and margin (2px 0px)

### Testing
- Add comprehensive test suite (61 tests total):
  - test_settings_ui.py: UI and basic integration (14 tests)
  - test_settings_dialog_structure.py: detailed structure validation (20 tests)
  - test_dialog_preferences_integration.py: DialogPreferences integration (14 tests)
  - test_delimiters_category.py: new Delimiters page validation (13 tests)
- Fix ruff issues (F841, S108) in test files
- All tests passing with 100% success rate

### Documentation
- Add docs/refactor_settings_ui.md: comprehensive refactor overview
- Add docs/delimiters_category.md: Delimiters feature documentation
- Add docs/ui_styles.md: dark theme style guide
- Add docs/testing_summary.md: testing overview and commands

### Code Quality
- Apply ruff/black/pyupgrade via pre-commit hooks
- Fix F841 (unused variable) in test_dialog_preferences_integration.py
- Fix S108 (insecure temp path) using Path.home() in test_settings_ui.py
- End-of-file fixes applied automatically

## Files Modified
- tidal_dl_ng/ui/dialog_settings.ui (complete restructure)
- tidal_dl_ng/ui/dialog_settings.py (auto-generated from .ui)
- tidal_dl_ng/dialog.py (add 4 delimiter params + Delimiters category)
- tests/* (4 new test files with 61 tests)
- docs/* (4 new documentation files)

## Impact
- Improved UX: intuitive category navigation, better visual organization
- Extensibility: easy to add new settings pages/categories
- Quality: comprehensive test coverage ensures robustness
- User flexibility: customizable separators for metadata and filenames

## Technical Notes
- QStackedWidget pages indexed 0-4 matching category list order
- Navigation via QListWidget::currentRowChanged signal
- Automatic save/load through existing DialogPreferences methods
- No breaking changes to existing settings or configuration

## Testing Commands
```powershell
poetry run pre-commit run -a
poetry run pytest -q

docs/delimiters_category.md

@@ -0,0 +1,43 @@
+# "Delimiters" Category
+
+## Goal
+
+Allow users to customize separators used for artists and album artists, both in metadata and filenames.
+
+## Parameters
+
+- `metadata_delimiter_artist`: artists separator (metadata)
+- `metadata_delimiter_album_artist`: album artists separator (metadata)
+- `filename_delimiter_artist`: artists separator (filename)
+- `filename_delimiter_album_artist`: album artists separator (filename)
+
+## UI
+
+- Page `page_delimiters` inside `sw_categories`
+- GroupBox `gb_delimiters`
+- 4 `QLineEdit` entries (`le_*`) with max width 100px
+
+## Integration
+
+- Add the 4 fields to `DialogPreferences.parameters_line_edit`
+- Load values via `populate_line_edit()`
+- Save via `to_settings()`
+
+## Tests
+
+- `tests/test_delimiters_category.py`: 13 tests
+  - Page existence
+  - Widgets accessibility
+  - Navigation to the page
+  - Width constraints
+
+## Examples
+
+- Semicolon: `"; "` → `Artist1; Artist2`
+- Ampersand: `" & "` → `Artist1 & Artist2`
+- Slash: `" / "` → `Artist1 / Artist2`
+
+## Notes
+
+- Default values are `", "` (comma + space)
+- Fields are kept short (max 100px) to avoid breaking the layout