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/app/scan.tsx

@@ -1,10 +1,12 @@
 import { CameraView, useCameraPermissions, type BarcodeScanningResult } from 'expo-camera';
 import { router } from 'expo-router';
-import { useRef, useState } from 'react';
+import { useEffect, useRef, useState } from 'react';
 import { Linking, StyleSheet, Text, View } from 'react-native';
 import { parseProfileLink } from '@musicshare/shared';
 import { Screen } from '@/components/Screen';
 import { Body, Button, Title } from '@/components/ui';
+import { FOSS_BUILD } from '@/lib/build';
+import { decodeQrFromJpeg, pickSnapshotSize } from '@/lib/qrSnapshot';
 import { useSession } from '@/lib/session';
 import { colors, radius, space } from '@/lib/theme';
 
@@ -15,8 +17,10 @@ export default function ScanScreen() {
   const [message, setMessage] = useState<string | null>(null);
   // The camera reports the same code many times a second; act on the first.
   const handled = useRef(false);
+  const camera = useRef<CameraView>(null);
+  const [snapshotSize, setSnapshotSize] = useState<string | undefined>();
 
-  const onScanned = ({ data }: BarcodeScanningResult) => {
+  const onCode = (data: string) => {
     if (handled.current) return;
     const username = parseProfileLink(data);
     if (!username) {
@@ -31,6 +35,36 @@ export default function ScanScreen() {
     router.replace(`/u/${username}`);
   };
 
+  // The snapshot loop calls whatever the current handler is.
+  const latest = useRef(onCode);
+  useEffect(() => {
+    latest.current = onCode;
+  });
+  const granted = Boolean(permission?.granted);
+
+  // F-Droid builds have no ML Kit: take a small snapshot every moment and decode it in JS.
+  useEffect(() => {
+    if (!FOSS_BUILD || !granted) return;
+    let stopped = false;
+    let timer: ReturnType<typeof setTimeout>;
+    const tick = async () => {
+      if (stopped || handled.current) return;
+      try {
+        const photo = await camera.current?.takePictureAsync({ base64: true, quality: 0.5, skipProcessing: true, shutterSound: false });
+        const data = photo?.base64 ? decodeQrFromJpeg(photo.base64) : null;
+        if (data) latest.current(data);
+      } catch {
+        // camera not ready yet, or a blurry frame; try again
+      }
+      if (!stopped) timer = setTimeout(tick, 600);
+    };
+    timer = setTimeout(tick, 800);
+    return () => {
+      stopped = true;
+      clearTimeout(timer);
+    };
+  }, [granted]);
+
   if (!permission) return <Screen />;
 
   if (!permission.granted) {
@@ -52,7 +86,18 @@ export default function ScanScreen() {
 
   return (
     <View style={styles.fill}>
-      <CameraView style={styles.fill} facing="back" barcodeScannerSettings={{ barcodeTypes: ['qr'] }} onBarcodeScanned={onScanned} />
+      {FOSS_BUILD ? (
+        <CameraView
+          ref={camera}
+          style={styles.fill}
+          facing="back"
+          pictureSize={snapshotSize}
+          animateShutter={false}
+          onCameraReady={async () => setSnapshotSize(pickSnapshotSize((await camera.current?.getAvailablePictureSizesAsync()) ?? []))}
+        />
+      ) : (
+        <CameraView style={styles.fill} facing="back" barcodeScannerSettings={{ barcodeTypes: ['qr'] }} onBarcodeScanned={({ data }: BarcodeScanningResult) => onCode(data)} />
+      )}
       <View style={styles.overlay} pointerEvents="box-none">
         <View style={styles.frame} />
         <Text style={styles.prompt}>{message ?? 'Point at a friend’s musicshare code'}</Text>