feat(integration): extract SillyTavern event handlers to dedicated module

- Create src/systems/integration/sillytavern.js with all event handlers
- Move commitTrackerData() (deferred from Epic 1)
- Move sendPlotProgression() to plotProgression.js
- Move updateGenerationModeUI() to layout.js
- Add registerAllEvents() and unregisterAllEvents() to events.js
- Centralize event registration in index.js initialization

This completes Epic 6: Integration Layer Extraction
~340 lines extracted from index.js
index.js reduced from ~783 lines to 423 lines

src/systems/ui/layout.js

@@ -252,3 +252,16 @@ export function applyPanelPosition() {
     // Update collapse toggle icon direction for new position
     updateCollapseToggleIcon();
 }
+
+/**
+ * Updates the UI based on generation mode selection.
+ */
+export function updateGenerationModeUI() {
+    if (extensionSettings.generationMode === 'together') {
+        // In "together" mode, manual update button is hidden
+        $('#rpg-manual-update').hide();
+    } else {
+        // In "separate" mode, manual update button is visible
+        $('#rpg-manual-update').show();
+    }
+}