Improve login UX for unconfirmed email + resend confirmation

- LoginForm: detect 403 separately, show amber warning with email address
- Add 'Resend confirmation email' button (calls POST /api/auth/resend-confirmation)
- Backend: POST /api/auth/resend-confirmation endpoint (generates new token, resends email)
- api.ts: add resendConfirmation method

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -161,6 +161,9 @@ export const api = {
 
     resetPassword: (token: string, password: string) =>
       req<{ ok: boolean }>('POST', '/api/auth/reset-password', { token, password }),
+
+    resendConfirmation: (email: string) =>
+      req<{ ok: boolean }>('POST', '/api/auth/resend-confirmation', { email }),
   },
 
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