feat(private-room): add image grid and swipe gallery

Render unlocked albums as adaptive nine-image grids and open the selected source image in the Gallery. Add pointer-driven swipe thresholds, edge resistance, animated snapping, adjacent-image loading, reduced-motion support, and coverage for filtering and navigation.

src/app/private-room/__tests__/private-album-gallery-motion.test.ts

@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  getResistedGalleryDragDistance,
+  resolveGallerySwipeDirection,
+} from "../private-album-gallery-motion";
+
+describe("private album Gallery motion", () => {
+  it("switches after crossing the viewport distance threshold", () => {
+    expect(
+      resolveGallerySwipeDirection({
+        distance: -72,
+        durationMs: 500,
+        viewportWidth: 390,
+        hasPrevious: true,
+        hasNext: true,
+      }),
+    ).toBe(1);
+    expect(
+      resolveGallerySwipeDirection({
+        distance: 72,
+        durationMs: 500,
+        viewportWidth: 390,
+        hasPrevious: true,
+        hasNext: true,
+      }),
+    ).toBe(-1);
+  });
+
+  it("switches on a short fast swipe but not a slow short drag", () => {
+    const baseInput = {
+      distance: -30,
+      viewportWidth: 390,
+      hasPrevious: true,
+      hasNext: true,
+    };
+
+    expect(
+      resolveGallerySwipeDirection({ ...baseInput, durationMs: 50 }),
+    ).toBe(1);
+    expect(
+      resolveGallerySwipeDirection({ ...baseInput, durationMs: 500 }),
+    ).toBe(0);
+  });
+
+  it("does not move beyond the first or last image", () => {
+    expect(
+      resolveGallerySwipeDirection({
+        distance: 100,
+        durationMs: 100,
+        viewportWidth: 390,
+        hasPrevious: false,
+        hasNext: true,
+      }),
+    ).toBe(0);
+    expect(
+      resolveGallerySwipeDirection({
+        distance: -100,
+        durationMs: 100,
+        viewportWidth: 390,
+        hasPrevious: true,
+        hasNext: false,
+      }),
+    ).toBe(0);
+  });
+
+  it("applies resistance only while pulling past an edge", () => {
+    expect(getResistedGalleryDragDistance(100, false, true)).toBeCloseTo(28);
+    expect(getResistedGalleryDragDistance(-100, true, false)).toBeCloseTo(-28);
+    expect(getResistedGalleryDragDistance(-100, false, true)).toBe(-100);
+  });
+});