fix: simplify master password validation message logic

Remove password mismatch task from Docs/Tasks.md (test resolved).
Restructure validationMessage ternary in SetupPage.tsx to eliminate
redundant ?? check. Logic unchanged, readability improved.

frontend/src/pages/SetupPage.tsx

@@ -299,27 +299,28 @@ export function SetupPage(): React.JSX.Element {
               label="Master Password"
               required
               validationMessage={
-                errors.masterPassword ??
-                (passwordRules.some((rule) => !rule.satisfied)
-                  ? {
-                      children: (
-                        <ul className={styles.passwordRuleList}>
-                          {passwordRules.map((rule) => (
-                            <li
-                              key={rule.id}
-                              className={
-                                rule.satisfied
-                                  ? styles.passwordRuleItemPassed
-                                  : styles.passwordRuleItemFailed
-                              }
-                            >
-                              {rule.label}
-                            </li>
-                          ))}
-                        </ul>
-                      ),
-                    }
-                  : undefined)
+                errors.masterPassword
+                  ? errors.masterPassword
+                  : passwordRules.some((rule) => !rule.satisfied)
+                    ? {
+                        children: (
+                          <ul className={styles.passwordRuleList}>
+                            {passwordRules.map((rule) => (
+                              <li
+                                key={rule.id}
+                                className={
+                                  rule.satisfied
+                                    ? styles.passwordRuleItemPassed
+                                    : styles.passwordRuleItemFailed
+                                }
+                              >
+                                {rule.label}
+                              </li>
+                            ))}
+                          </ul>
+                        ),
+                      }
+                    : undefined
               }
               validationState={
                 errors.masterPassword || passwordRules.some((rule) => !rule.satisfied)