`feat(level-select): open on the player's current campaign by default`

Instead of always starting at campaign 1 (index 0), `LevelSelectScene` now initializes to the campaign containing the first uncleared level in game order, so re-entering the map lands you where you left off. Falls back to campaign 1 if everything is cleared; empty campaigns are skipped since they can't hold the current level.

src/scenes/LevelSelectScene.js

@@ -61,7 +61,7 @@ export default class LevelSelectScene extends Phaser.Scene {
     // shown; it handles first-time start, campaign-to-campaign swaps, and
     // re-entry from the main menu alike.
 
-    this.campaignIndex = 0;
+    this.campaignIndex = this._defaultCampaignIndex();
     this._groups = { bg: [], path: [], nodes: [], tags: [] };
     this._transitioning = false;
     this._pulseTween = null;
@@ -108,6 +108,18 @@ export default class LevelSelectScene extends Phaser.Scene {
 
   // ------------------------------------------------------------------ build
 
+  // Enter on the campaign the player is currently on: the one containing the
+  // first uncleared level in game order (levels unlock strictly in that order,
+  // so the campaign holding it is always a campaign that is unlocked). If
+  // everything is cleared, fall back to campaign 1 (index 0). An empty
+  // campaign can't hold the current level, so it's skipped.
+  _defaultCampaignIndex() {
+    for (let i = 0; i < CAMPAIGNS.length; i++) {
+      if (CAMPAIGNS[i].levels.some((l) => !isLevelComplete(l.id))) return i;
+    }
+    return 0;
+  }
+
   _buildCampaign(instant = false) {
     for (const key of Object.keys(this._groups)) {
       for (const obj of this._groups[key]) obj.destroy();