feat: enhance roleplay features with regex, exports, and validation

This commit implements Options B, C, and D for enhanced roleplay capabilities:

**Option B: Enhanced Roleplay Features**
- Add regex pattern matching for World Info keywords with fallback to literal matching
- Make scan depth configurable (default: 20, range: 1-100 messages)
- Make Author's Note insertion depth configurable (default: 3, range: 1-50 messages)
- Add World Info import/export functionality with merge/replace options
- Update WorldInfoEntry struct with use_regex field
- Update RoleplaySettings with scan_depth and authors_note_depth fields
- Modify build_roleplay_context() to return configurable note_depth
- Update all 4 chat functions to use configurable Author's Note depth

**Option C: Chat Export Enhancements**
- Add export_chat_as_markdown: Clean markdown format with bold role labels
- Add export_chat_as_text: Simple plain text format with decorative separator
- Add export_chat_as_html: Styled HTML with responsive design and color-coded messages
- All exports use native file dialogs with appropriate file filters

**Option D: Polish & Quality of Life**
- Add validate_regex_pattern command for real-time regex validation
- Add update_roleplay_depths command with range validation
- Enhance add_world_info_entry with input validation (regex, empty checks)
- Enhance update_world_info_entry with input validation
- Add detailed error messages for all validation failures

**Technical Details:**
- Add regex dependency to Cargo.toml
- Use #[serde(default)] attributes for backward compatibility
- Graceful error handling with fallback strategies
- HTML export includes proper escaping for XSS protection
- All new commands registered in Tauri invoke_handler

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -31,4 +31,5 @@ bytes = "1"
 png = "0.17"
 base64 = "0.21"
 image = "0.24"
+regex = "1"