Migrate from .NET MAUI to Expo + Convex

Complete rewrite of Scry using TypeScript stack:

- Expo/React Native with Expo Router (file-based routing)
- Convex backend (serverless functions + real-time database)
- Adaptive camera system (expo-camera in Expo Go, Vision Camera in production)
- React Native Skia + fast-opencv for image processing
- GDPR-compliant auth setup with Zitadel OIDC (pending configuration)

Key features:
- Card recognition pipeline ported to TypeScript
- Perceptual hashing (192-bit color pHash)
- CLAHE preprocessing for lighting normalization
- Local SQLite cache with Convex sync
- Collection management with offline support

Removes all .NET/MAUI code (src/, test/, tools/).

💘 Generated with Crush

Assisted-by: Claude Opus 4.5 via Crush <crush@charm.land>

lib/hooks/useAuth.ts

@@ -0,0 +1,112 @@
+/**
+ * Authentication hook and utilities.
+ * 
+ * NOTE: Auth is currently disabled (using ConvexProvider instead of ConvexAuthProvider).
+ * This hook provides a stub implementation until Zitadel is configured.
+ */
+
+import { useState, useCallback } from "react";
+
+/**
+ * Hook for authentication state and actions.
+ * Currently returns unauthenticated state - enable ConvexAuthProvider when Zitadel is ready.
+ */
+export function useAuth() {
+  const [error, setError] = useState<string | null>(null);
+
+  const handleSignIn = useCallback(async () => {
+    setError("Authentication not configured. Set up Zitadel environment variables.");
+  }, []);
+
+  const handleSignOut = useCallback(async () => {
+    // No-op when not authenticated
+  }, []);
+
+  return {
+    isAuthenticated: false,
+    isLoading: false,
+    error,
+    signIn: handleSignIn,
+    signOut: handleSignOut,
+    clearError: () => setError(null),
+  };
+}
+
+// Full implementation for when ConvexAuthProvider is enabled:
+/*
+import { useConvexAuth } from "convex/react";
+import { useAuthActions } from "@convex-dev/auth/react";
+import { makeRedirectUri } from "expo-auth-session";
+import { openAuthSessionAsync } from "expo-web-browser";
+import { Platform } from "react-native";
+
+const redirectUri = makeRedirectUri({
+  scheme: "scry",
+  path: "auth",
+});
+
+export function useAuth() {
+  const { isAuthenticated, isLoading } = useConvexAuth();
+  const { signIn, signOut } = useAuthActions();
+  const [isSigningIn, setIsSigningIn] = useState(false);
+  const [error, setError] = useState<string | null>(null);
+
+  const handleSignIn = useCallback(async () => {
+    setIsSigningIn(true);
+    setError(null);
+
+    try {
+      const result = await signIn("zitadel", { redirectTo: redirectUri });
+
+      if (Platform.OS === "web") {
+        return;
+      }
+
+      if (result?.redirect) {
+        const authResult = await openAuthSessionAsync(
+          result.redirect.toString(),
+          redirectUri,
+          {
+            showInRecents: true,
+            preferEphemeralSession: false,
+          }
+        );
+
+        if (authResult.type === "success") {
+          const url = new URL(authResult.url);
+          const code = url.searchParams.get("code");
+
+          if (code) {
+            await signIn("zitadel", { code });
+          }
+        } else if (authResult.type === "cancel") {
+          setError("Sign-in was cancelled");
+        }
+      }
+    } catch (err) {
+      console.error("Sign-in error:", err);
+      setError(err instanceof Error ? err.message : "Sign-in failed");
+    } finally {
+      setIsSigningIn(false);
+    }
+  }, [signIn]);
+
+  const handleSignOut = useCallback(async () => {
+    try {
+      await signOut();
+    } catch (err) {
+      console.error("Sign-out error:", err);
+      setError(err instanceof Error ? err.message : "Sign-out failed");
+    }
+  }, [signOut]);
+
+  return {
+    isAuthenticated,
+    isLoading: isLoading || isSigningIn,
+    error,
+    signIn: handleSignIn,
+    signOut: handleSignOut,
+    clearError: () => setError(null),
+  };
+}
+*/