refactor(core): extract core modules (state, persistence, config, events)

Extract core system modules from monolithic index.js into modular architecture:

- src/core/state.js: All extension state variables with controlled setters
- src/core/persistence.js: Settings and chat data persistence functions
- src/core/config.js: Extension metadata and default configuration
- src/core/events.js: SillyTavern event system wrapper

Updated index.js to import and use new core modules.
Removed ~220 lines of state/persistence code from index.js.

Part of Epic 1: Foundation & Core Systems (Phase 1.1-1.2)

src/core/config.js

@@ -0,0 +1,66 @@
+/**
+ * Core Configuration Module
+ * Extension metadata and configuration constants
+ */
+
+export const extensionName = 'third-party/rpg-companion-sillytavern';
+
+/**
+ * Dynamically determine extension path based on current location
+ * This supports both global (public/extensions) and user-specific (data/default-user/extensions) installations
+ */
+const currentScriptPath = import.meta.url;
+const isUserExtension = currentScriptPath.includes('/data/') || currentScriptPath.includes('\\data\\');
+export const extensionFolderPath = isUserExtension
+    ? `data/default-user/extensions/${extensionName}`
+    : `scripts/extensions/${extensionName}`;
+
+/**
+ * Default extension settings
+ */
+export const defaultSettings = {
+    enabled: true,
+    autoUpdate: true,
+    updateDepth: 4, // How many messages to include in the context
+    generationMode: 'together', // 'separate' or 'together' - whether to generate with main response or separately
+    showUserStats: true,
+    showInfoBox: true,
+    showCharacterThoughts: true,
+    showThoughtsInChat: true, // Show thoughts overlay in chat
+    enableHtmlPrompt: false, // Enable immersive HTML prompt injection
+    enablePlotButtons: true, // Show plot progression buttons above chat input
+    panelPosition: 'right', // 'left', 'right', or 'top'
+    theme: 'default', // Theme: default, sci-fi, fantasy, cyberpunk, custom
+    customColors: {
+        bg: '#1a1a2e',
+        accent: '#16213e',
+        text: '#eaeaea',
+        highlight: '#e94560'
+    },
+    statBarColorLow: '#cc3333', // Color for low stat values (red)
+    statBarColorHigh: '#33cc66', // Color for high stat values (green)
+    enableAnimations: true, // Enable smooth animations for stats and content updates
+    mobileFabPosition: {
+        top: 'calc(var(--topBarBlockSize) + 60px)',
+        right: '12px'
+    }, // Saved position for mobile FAB button
+    userStats: {
+        health: 100,
+        satiety: 100,
+        energy: 100,
+        hygiene: 100,
+        arousal: 0,
+        mood: '😐',
+        conditions: 'None',
+        inventory: 'None'
+    },
+    classicStats: {
+        str: 10,
+        dex: 10,
+        con: 10,
+        int: 10,
+        wis: 10,
+        cha: 10
+    },
+    lastDiceRoll: null // Store last dice roll result
+};