The web's password forms read the server's minimum length

The server's MIN_PASSWORD_LEN was 8, and the web wrote 8 out five times: two
checks and three placeholders. The constant moves beside the other policy numbers
in settings.py (auth.py imports it), /api/config serves it as
min_password_length, and the config store hands it to Register, Reset and
Account. 8 stays only as the fallback until the config answers.

DRY pass #2, batch 3 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/views/RegisterView.vue

@@ -24,8 +24,8 @@ const loading = ref(false);
 
 async function submit() {
   error.value = "";
-  if (password.value.length < 8) {
-    error.value = "Password must be at least 8 characters.";
+  if (password.value.length < config.minPasswordLength) {
+    error.value = `Password must be at least ${config.minPasswordLength} characters.`;
     return;
   }
   loading.value = true;
@@ -80,7 +80,7 @@ async function submit() {
           label="Password"
           type="password"
           autocomplete="new-password"
-          placeholder="At least 8 characters"
+          :placeholder="`At least ${config.minPasswordLength} characters`"
           required
         />
         <p