merge: integrate upstream tracker customization system

Merged upstream/main (82b9564) which includes:
- Full tracker customization system
- Tracker editor UI component
- Custom stat names in AI prompts
- Multi-line tracker format updates

Conflict resolutions:
- src/core/persistence.js: Kept both dashboard v2 and trackerConfig migrations
- style.css: Accepted upstream responsive calendar styling with clamp()

Both migration systems are now active and will run in sequence.

index.js

@@ -86,6 +86,9 @@ import {
     addDiceQuickReply,
     getSettingsModal
 } from './src/systems/ui/modals.js';
+import {
+    initTrackerEditor
+} from './src/systems/ui/trackerEditor.js';
 import {
     togglePlotButtons,
     updateCollapseToggleIcon,
@@ -513,6 +516,7 @@ async function initUI() {
     setupDiceRoller();
     setupClassicStatsButtons();
     setupSettingsPopup();
+    initTrackerEditor();
     addDiceQuickReply();
     setupPlotButtons(sendPlotProgression);
     setupMobileKeyboardHandling();