feat(auth): add TOTP two-factor authentication across all apps

Uses Better Auth's built-in twoFactor plugin for TOTP + backup codes:

Backend (mana-core-auth):
- twoFactor plugin in better-auth.config.ts (issuer: ManaCore)
- twoFactorEnabled field on users table, backupCodes as encrypted text
- 2FA redirect detection in signIn flow
- Passthrough controller forwards /two-factor/* to Better Auth
- Security event types for 2FA operations

Client (shared-auth):
- enableTwoFactor, disableTwoFactor, verifyTwoFactor, verifyBackupCode,
  generateBackupCodes methods with session-to-token exchange

UI (shared-auth-ui):
- LoginPage: 2FA code input view after password login, backup code toggle
- TwoFactorSetup: settings component with enable/disable/QR code/backup codes

App integration:
- All 19 auth stores have verifyTwoFactor() and verifyBackupCode()
- All 19 login pages pass onVerifyTwoFactor and onVerifyBackupCode callbacks
- ManaCore settings page has TwoFactorSetup component

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

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

@@ -117,6 +117,28 @@ export const authStore = {
 		}
 	},
 
+	async verifyTwoFactor(code: string, trustDevice?: boolean) {
+		const authService = getAuthService();
+		if (!authService) return { success: false, error: 'Auth not available on server' };
+		const result = await authService.verifyTwoFactor(code, trustDevice);
+		if (result.success) {
+			const userData = await authService.getUserFromToken();
+			user = userData;
+		}
+		return result;
+	},
+
+	async verifyBackupCode(code: string) {
+		const authService = getAuthService();
+		if (!authService) return { success: false, error: 'Auth not available on server' };
+		const result = await authService.verifyBackupCode(code);
+		if (result.success) {
+			const userData = await authService.getUserFromToken();
+			user = userData;
+		}
+		return result;
+	},
+
 	isPasskeyAvailable(): boolean {
 		const authService = getAuthService();
 		if (!authService) return false;

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

@@ -60,6 +60,8 @@
 	onResendVerification={handleResendVerification}
 	passkeyAvailable={authStore.isPasskeyAvailable()}
 	onSignInWithPasskey={() => authStore.signInWithPasskey()}
+	onVerifyTwoFactor={(code, trust) => authStore.verifyTwoFactor(code, trust)}
+	onVerifyBackupCode={(code) => authStore.verifyBackupCode(code)}
 	{goto}
 	successRedirect={redirectTo}
 	registerPath="/register"