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.ts

@@ -0,0 +1,50 @@
+// Shared byte-size formatter for the Downloads surface.
+//
+// One formatter, used everywhere disk usage is shown (cards, detail pages, the
+// device total, and the remove-reclaim prompt) so units are consistent. We use
+// DECIMAL units (1 GB = 1000 MB), matching how phone storage screens and file
+// browsers present sizes, and show 2–3 significant figures.
+//
+// TRACES: UR-056 | DR-085
+
+const UNITS = ["B", "KB", "MB", "GB", "TB", "PB"] as const;
+
+/**
+ * Format a byte count as a human-readable size string (decimal units).
+ *
+ * Examples: 0 → "0 B", 340_000_000 → "340 MB", 1_200_000_000 → "1.2 GB".
+ *
+ * - Bytes render as whole numbers (no "0.5 B").
+ * - KB and above show enough decimals for 2–3 significant figures: values
+ *   ≥ 100 render with no decimals, ≥ 10 with one, otherwise two.
+ * - Negative / non-finite inputs are treated as 0 (sizes are never negative).
+ */
+export function formatBytes(bytes: number): string {
+  if (!Number.isFinite(bytes) || bytes <= 0) return "0 B";
+
+  let value = bytes;
+  let unitIndex = 0;
+  while (value >= 1000 && unitIndex < UNITS.length - 1) {
+    value /= 1000;
+    unitIndex += 1;
+  }
+
+  // Bytes are always whole; larger units get 2–3 significant figures.
+  let formatted: string;
+  if (unitIndex === 0) {
+    formatted = Math.round(value).toString();
+  } else if (value >= 100) {
+    formatted = Math.round(value).toString();
+  } else if (value >= 10) {
+    formatted = value.toFixed(1);
+  } else {
+    formatted = value.toFixed(2);
+  }
+
+  // Trim trailing zeros ("1.20" → "1.2", "1.00" → "1") for a cleaner label.
+  if (formatted.includes(".")) {
+    formatted = formatted.replace(/\.?0+$/, "");
+  }
+
+  return `${formatted} ${UNITS[unitIndex]}`;
+}