feat(downloads): browsable downloaded library with on-disk usage

Replace the flat download list with a Downloaded browse surface that
reuses the online grids/cards/detail pages, filtered to on-device media,
plus a demoted Transfers tab. Add repository browse commands
(getDownloadedLibraries/Items, disk usage) with offline/hybrid
implementations, a downloadedCatalog service, formatBytes helper, and
per-item/device disk-usage labels on cards and grids. Regenerated
bindings.

Also carries the inseparable UR-052 offline-filter hunks in
offline.rs/hybrid.rs.

TRACES: UR-055 | DR-081, DR-082, DR-083, DR-084; UR-056 | DR-085

src/lib/utils/formatBytes.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from "vitest";
+import { formatBytes } from "./formatBytes";
+
+// TRACES: UR-056 | DR-085 | UT-050
+describe("formatBytes", () => {
+  it("renders zero and non-positive as '0 B'", () => {
+    expect(formatBytes(0)).toBe("0 B");
+    expect(formatBytes(-1)).toBe("0 B");
+    expect(formatBytes(NaN)).toBe("0 B");
+    expect(formatBytes(Infinity)).toBe("0 B");
+  });
+
+  it("renders whole bytes below 1 KB", () => {
+    expect(formatBytes(1)).toBe("1 B");
+    expect(formatBytes(999)).toBe("999 B");
+  });
+
+  it("crosses to KB at 1000 bytes (decimal units)", () => {
+    expect(formatBytes(1000)).toBe("1 KB");
+    expect(formatBytes(1500)).toBe("1.5 KB");
+  });
+
+  it("shows 2-3 significant figures", () => {
+    expect(formatBytes(340_000_000)).toBe("340 MB");
+    expect(formatBytes(1_200_000_000)).toBe("1.2 GB");
+    expect(formatBytes(48_000_000)).toBe("48 MB");
+  });
+
+  it("trims trailing zeros", () => {
+    expect(formatBytes(2_000_000_000)).toBe("2 GB");
+    expect(formatBytes(10_000_000)).toBe("10 MB");
+  });
+
+  it("scales into large units", () => {
+    expect(formatBytes(3_400_000_000)).toBe("3.4 GB");
+    expect(formatBytes(1_000_000_000_000)).toBe("1 TB");
+  });
+
+  it("uses no decimals at or above 100 of a unit", () => {
+    // 123.4 MB → "123 MB" (2-3 sig figs, whole number band)
+    expect(formatBytes(123_400_000)).toBe("123 MB");
+  });
+});