Get the repo ready for F-Droid

- License: AGPL-3.0-or-later (LICENSE, every package.json, README).
- MUSICSHARE_FOSS=1 builds leave out the only closed-source code, Google's
  ML Kit / Play Services barcode scanner bundled by expo-camera; the scan
  screen then decodes small camera snapshots with jsQR instead.
- Expo modules build from source rather than from precompiled AARs, as
  F-Droid requires (that's also what makes the scanner switch take effect).
  Verified: a FOSS build contains no ML Kit or Play Services classes.
- Release builds default to the production server, and come out unsigned
  without our upload key (F-Droid signs with its own). versionCode 1.
- Store listing in fastlane/metadata/android, a draft fdroiddata recipe in
  fdroid/, and docs/FDROID.md with the submission steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MzFywujuH9TiuXbr9vjoX8

apps/mobile/src/lib/qrSnapshot.ts

@@ -0,0 +1,25 @@
+import jpeg from 'jpeg-js';
+import jsQR from 'jsqr';
+
+/**
+ * Reads a QR code from a small JPEG snapshot (base64), entirely in JS. Used by
+ * F-Droid builds, which leave out Google's ML Kit scanner. Returns the code's
+ * text, or null if there isn't one.
+ */
+export function decodeQrFromJpeg(base64: string): string | null {
+  const bytes = Uint8Array.from(atob(base64), (c) => c.charCodeAt(0));
+  const image = jpeg.decode(bytes, { useTArray: true, formatAsRGBA: true, maxResolutionInMP: 4 });
+  const code = jsQR(new Uint8ClampedArray(image.data.buffer, image.data.byteOffset, image.data.length), image.width, image.height, {
+    inversionAttempts: 'dontInvert',
+  });
+  return code?.data ?? null;
+}
+
+/** The smallest snapshot size that still reads a QR code held at arm's length. */
+export function pickSnapshotSize(sizes: string[]): string | undefined {
+  const parsed = sizes
+    .map((s) => ({ s, w: Number(s.split('x')[0]), h: Number(s.split('x')[1]) }))
+    .filter((p) => p.w > 0 && p.h > 0)
+    .sort((a, b) => a.w * a.h - b.w * b.h);
+  return (parsed.find((p) => Math.min(p.w, p.h) >= 480) ?? parsed.at(-1))?.s;
+}