feat(auth): add WebAuthn/Passkey support across all apps

Implements passwordless authentication via passkeys using @simplewebauthn:

Backend (mana-core-auth):
- New passkeys table in auth schema (credentialId, publicKey, counter, etc.)
- PasskeyService with registration/authentication flows and challenge storage
- 7 new API endpoints (register, authenticate, list, delete, rename)
- createSessionAndTokens helper for non-password auth flows
- Security event types for passkey operations

Client (shared-auth):
- signInWithPasskey() and registerPasskey() with dynamic @simplewebauthn/browser imports
- isPasskeyAvailable() browser capability check
- Passkey management methods (list, delete, rename)

UI (shared-auth-ui):
- Passkey button on LoginPage with key icon, shown when browser supports WebAuthn
- Divider between passkey and email/password form

App integration:
- All 19 web app auth stores have isPasskeyAvailable() and signInWithPasskey()
- All 19 web app login pages pass passkeyAvailable and onSignInWithPasskey props
- rpID=mana.how in production enables cross-app passkey usage (SSO-compatible)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/questions/apps/web/src/lib/stores/auth.svelte.ts

@@ -113,6 +113,35 @@ export const authStore = {
 		}
 	},
 
+	isPasskeyAvailable(): boolean {
+		const authService = getAuthService();
+		if (!authService) return false;
+		return authService.isPasskeyAvailable();
+	},
+
+	async signInWithPasskey() {
+		const authService = getAuthService();
+		if (!authService) {
+			return { success: false, error: 'Auth not available on server' };
+		}
+
+		try {
+			const result = await authService.signInWithPasskey();
+
+			if (!result.success) {
+				return { success: false, error: result.error || 'Passkey authentication failed' };
+			}
+
+			const userData = await authService.getUserFromToken();
+			user = userData;
+
+			return { success: true };
+		} catch (error) {
+			const errorMessage = error instanceof Error ? error.message : 'Unknown error';
+			return { success: false, error: errorMessage };
+		}
+	},
+
 	async signIn(email: string, password: string) {
 		const authService = getAuthService();
 		if (!authService) {

apps/questions/apps/web/src/routes/(auth)/login/+page.svelte

@@ -58,6 +58,8 @@
 	primaryColor="#8b5cf6"
 	onSignIn={handleSignIn}
 	onResendVerification={handleResendVerification}
+	passkeyAvailable={authStore.isPasskeyAvailable()}
+	onSignInWithPasskey={() => authStore.signInWithPasskey()}
 	{goto}
 	successRedirect={redirectTo}
 	registerPath="/register"