Add drop shadow system with animated lift/drop during drag

DropShadowFilter applied to all ImageSprites for photos-on-a-desk feel.
SelectionManager now handles object dragging with shadow lift/drop and
spring-animated scale (1.0 <-> 1.03). Replaced incompatible @pixi/filter-drop-shadow
v5 with pixi-filters v6 for PixiJS v8 compatibility.

frontend/src/canvas/sprites/ImageSprite.ts

@@ -1,4 +1,5 @@
 import { Sprite, Texture, Graphics } from "pixi.js";
+import { DropShadowFilter } from "pixi-filters";
 import { TextureManager, LODTier } from "../TextureManager";
 
 /**
@@ -6,8 +7,14 @@ import { TextureManager, LODTier } from "../TextureManager";
  * Shows a placeholder shimmer rect until the first texture tier loads,
  * then swaps textures as zoom level changes.
  */
+// Shadow defaults (resting state)
+const SHADOW_REST = { offsetX: 3, offsetY: 3, blur: 4, alpha: 0.25 };
+// Shadow lifted state (during drag)
+const SHADOW_LIFT = { offsetX: 6, offsetY: 8, blur: 8, alpha: 0.35 };
+
 export class ImageSprite extends Sprite {
   readonly assetKey: string;
+  readonly shadow: DropShadowFilter;
 
   private textures: TextureManager;
   private currentTier: LODTier | null = null;
@@ -32,6 +39,15 @@ export class ImageSprite extends Sprite {
     this.width = w;
     this.height = h;
 
+    // Drop shadow for photos-on-a-desk feel
+    this.shadow = new DropShadowFilter({
+      offset: { x: SHADOW_REST.offsetX, y: SHADOW_REST.offsetY },
+      blur: SHADOW_REST.blur,
+      alpha: SHADOW_REST.alpha,
+      color: 0x000000,
+    });
+    this.filters = [this.shadow];
+
     // Create placeholder: dark rect shown until first texture loads
     const placeholder = new Graphics();
     placeholder.rect(0, 0, w, h).fill(0x2a2a2a);
@@ -42,6 +58,20 @@ export class ImageSprite extends Sprite {
     this.loadTier("thumb");
   }
 
+  /** Expand shadow for drag-lift effect. */
+  liftShadow(): void {
+    this.shadow.offset = { x: SHADOW_LIFT.offsetX, y: SHADOW_LIFT.offsetY };
+    this.shadow.blur = SHADOW_LIFT.blur;
+    this.shadow.alpha = SHADOW_LIFT.alpha;
+  }
+
+  /** Restore shadow to resting state. */
+  dropShadow(): void {
+    this.shadow.offset = { x: SHADOW_REST.offsetX, y: SHADOW_REST.offsetY };
+    this.shadow.blur = SHADOW_REST.blur;
+    this.shadow.alpha = SHADOW_REST.alpha;
+  }
+
   /**
    * Load a specific LOD tier texture for this sprite.
    * Skips if already at the requested tier or currently loading.