feat(createPerspectiveManager): create perspective manager to work with perspective, moving objects along the z axis

src/shared/lib/helpers/createPerspectiveManager/createPerspectiveManager.svelte.ts

@@ -0,0 +1,130 @@
+import { Spring } from 'svelte/motion';
+
+export interface PerspectiveConfig {
+    /**
+     * How many px to move back per level
+     */
+    depthStep?: number;
+    /**
+     * Scale reduction per level
+     */
+    scaleStep?: number;
+    /**
+     * Blur amount per level
+     */
+    blurStep?: number;
+    /**
+     * Opacity reduction per level
+     */
+    opacityStep?: number;
+    /**
+     * Parallax intensity per level
+     */
+    parallaxIntensity?: number;
+    /**
+     * Horizontal offset for each plan (x-axis positioning)
+     * Positive = right, Negative = left
+     */
+    horizontalOffset?: number;
+    /**
+     * Layout mode: 'center' (default) or 'split' for Swiss-style side-by-side
+     */
+    layoutMode?: 'center' | 'split';
+}
+
+/**
+ * Manages perspective state with a simple boolean flag.
+ *
+ * Drastically simplified from the complex camera/index system.
+ * Just manages whether content is in "back" or "front" state.
+ *
+ * @example
+ * ```typescript
+ * const perspective = createPerspectiveManager({
+ *     depthStep: 100,
+ *     scaleStep: 0.5,
+ *     blurStep: 4,
+ * });
+ *
+ * // Toggle back/front
+ * perspective.toggle();
+ *
+ * // Check state
+ * const isBack = perspective.isBack; // reactive boolean
+ * ```
+ */
+export class PerspectiveManager {
+    /**
+     * Spring for smooth back/front transitions
+     */
+    spring = new Spring(0, {
+        stiffness: 0.2,
+        damping: 0.8,
+    });
+
+    /**
+     * Reactive boolean: true when in back position (blurred, scaled down)
+     */
+    isBack = $derived(this.spring.current > 0.5);
+
+    /**
+     * Reactive boolean: true when in front position (fully visible, interactive)
+     */
+    isFront = $derived(this.spring.current < 0.5);
+
+    /**
+     * Configuration values for style computation
+     */
+    private config: Required<PerspectiveConfig>;
+
+    constructor(config: PerspectiveConfig = {}) {
+        this.config = {
+            depthStep: config.depthStep ?? 100,
+            scaleStep: config.scaleStep ?? 0.5,
+            blurStep: config.blurStep ?? 4,
+            opacityStep: config.opacityStep ?? 0.5,
+            parallaxIntensity: config.parallaxIntensity ?? 0,
+            horizontalOffset: config.horizontalOffset ?? 0,
+            layoutMode: config.layoutMode ?? 'center',
+        };
+    }
+
+    /**
+     * Toggle between front (0) and back (1) positions.
+     * Smooth spring animation handles the transition.
+     */
+    toggle = () => {
+        const target = this.spring.current < 0.5 ? 1 : 0;
+        this.spring.target = target;
+    };
+
+    /**
+     * Force to back position
+     */
+    setBack = () => {
+        this.spring.target = 1;
+    };
+
+    /**
+     * Force to front position
+     */
+    setFront = () => {
+        this.spring.target = 0;
+    };
+
+    /**
+     * Get configuration for style computation
+     * @internal
+     */
+    getConfig = () => this.config;
+}
+
+/**
+ * Factory function to create a PerspectiveManager instance.
+ *
+ * @param config - Configuration options
+ * @returns Configured PerspectiveManager instance
+ */
+export function createPerspectiveManager(config: PerspectiveConfig = {}) {
+    return new PerspectiveManager(config);
+}