Add customizable RPG attributes and fix character stats editing

Features:
- Made RPG attributes (STR/DEX/CON/INT/WIS/CHA) fully customizable
- Added enable/disable toggle for entire RPG Attributes section
- Users can add/remove/rename/toggle individual attributes
- Custom attribute names now appear in AI prompts for dice rolls
- Added proper CSS styling for attribute editor fields

Bug Fixes:
- Fixed character stat editing showing 0% on blur but saving correctly
- Character stats now create Stats line if missing from AI response
- Separated stat name from editable percentage value
- Added value sanitization (removes %, validates 0-100 range)
- Stats line now inserts before Thoughts line when created

Technical:
- Added buildAttributesString() helper in promptBuilder.js
- Updated generateTrackerInstructions and generateContextualSummary
- Restructured character stat HTML to prevent nested contenteditable
- Enhanced updateCharacterField to handle missing Stats lines
- Removed legacy default preset/regex import code

src/core/state.js

@@ -72,6 +72,7 @@ export let extensionSettings = {
                 { id: 'arousal', name: 'Arousal', enabled: true }
             ],
             // RPG Attributes (customizable D&D-style attributes)
+            showRPGAttributes: true,
             rpgAttributes: [
                 { id: 'str', name: 'STR', enabled: true },
                 { id: 'dex', name: 'DEX', enabled: true },