feat: integrate settings validation API before saving upload path

- saveUploadPath now validates path via POST /api/v2/settings/validate
- Shows clear error message if path doesn't exist or isn't writable
- Only saves if validation passes (valid: true)
- 5 validation checks: format, existence, type, read/write permissions

src/views/AdminView.vue

@@ -1104,6 +1104,21 @@ async function saveUploadPath() {
   saveMessage.value = ''
   saveError.value = false
   try {
+    // First validate the path
+    const validateResp = await fetch(`${markbaseUrl.value}/api/v2/settings/validate`, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ server: { upload_path: uploadPath.value } })
+    })
+    const validateData = await validateResp.json()
+    if (!validateData.valid) {
+      saveError.value = true
+      saveMessage.value = validateData.message || 'Path validation failed'
+      saving.value = false
+      return
+    }
+
+    // Path is valid, now save
     const resp = await fetch(`${markbaseUrl.value}/api/v2/settings`, {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },