Add user profile panel with self-service password change and dark theme UserMenu

backend/__tests__/auth-password-change.property.test.js

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+/**
+ * Property-Based Test: Password Change Round-Trip
+ *
+ * Feature: user-profile, Property 3: Password change round-trip
+ *
+ * For any valid current password and any new password of 8+ characters,
+ * after a successful change, bcrypt.compare(newPassword, storedHash) returns true.
+ *
+ * Validates: Requirements 2.2, 2.7
+ */
+
+const fc = require('fast-check');
+const bcrypt = require('bcryptjs');
+
+// bcrypt cost factor — production uses 10, but we use 4 (the minimum) here
+// to keep 100 iterations feasible within test timeouts. The round-trip property
+// holds regardless of cost factor.
+const BCRYPT_COST = 4;
+
+describe('Feature: user-profile, Property 3: Password change round-trip', () => {
+  it('after a password change, bcrypt.compare(newPassword, newHash) returns true', async () => {
+    await fc.assert(
+      fc.asyncProperty(
+        // Current password: any non-empty string (length >= 1)
+        fc.string({ minLength: 1, maxLength: 72 }),
+        // New password: any string of length >= 8 (bcrypt max input is 72 bytes)
+        fc.string({ minLength: 8, maxLength: 72 }),
+        async (currentPassword, newPassword) => {
+          // Step 1: Hash the current password (simulates existing stored hash)
+          const currentHash = await bcrypt.hash(currentPassword, BCRYPT_COST);
+
+          // Step 2: Verify the current password against the stored hash
+          // (simulates the bcrypt.compare check in the change-password route)
+          const currentPasswordValid = await bcrypt.compare(currentPassword, currentHash);
+          expect(currentPasswordValid).toBe(true);
+
+          // Step 3: Hash the new password (simulates bcrypt.hash(newPassword, 10) in the route)
+          const newHash = await bcrypt.hash(newPassword, BCRYPT_COST);
+
+          // Step 4: Verify the new password matches the new hash (round-trip property)
+          const newPasswordValid = await bcrypt.compare(newPassword, newHash);
+          expect(newPasswordValid).toBe(true);
+        }
+      ),
+      { numRuns: 100 }
+    );
+  }, 120000); // 2-minute timeout for 100 bcrypt iterations
+});