Add mission briefing videos for all 5 missions

- Add binary assets: `assets/video/mission_001.mp4` through `mission_005.mp4`
- Update `CampaignScene.preload()` to dynamically load all mission videos
- Modify briefing display logic to show the loaded video when available, hiding offline/file placeholders

src/scenes/CampaignScene.js

@@ -54,6 +54,11 @@ export class CampaignScene extends Phaser.Scene {
 
   preload() {
     this.load.video('corridors', 'assets/video/corridors.mp4');
+    // Load mission briefing videos
+    for (let i = 1; i <= 5; i++) {
+      const id = `mission_${String(i).padStart(3, '0')}`;
+      this.load.video(id, `assets/video/${id}.mp4`);
+    }
   }
 
   create() {
@@ -449,11 +454,9 @@ export class CampaignScene extends Phaser.Scene {
     this._termContainer.add(fileText);
     this._briefingElements.push(fileText);
 
-    // Attempt to load actual video
-    const vidKey = `vid_mission_${mission.id}`;
+    // Show actual video if loaded
     try {
       if (this.cache.video.has(mission.id)) {
-        // If video was previously loaded, show it
         offlineText.setVisible(false);
         fileText.setVisible(false);
         const vid = this.add.video(vidX, vidY, mission.id);