Fix quota endpoint: path detection, error handling, better messages

- Backend: detect the actual filesystem mountpoint via 'df' instead of
  hardcoding /srv/samba — setquota needs the mountpoint, not a subpath
- Backend: catch CalledProcessError and FileNotFoundError from setquota
  and return the actual error message (e.g. quota not enabled, package
  not installed) instead of crashing with 500
- Frontend: always send the share path in the quota API call so the
  backend can find the right filesystem, even for SMB per-user quotas
- Frontend: read the actual error message from the response and show it
  in the toast instead of just "N quota(s) failed"

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

app.py

@@ -417,14 +417,23 @@ def toggle_nfs():
 
 # ===== QUOTA =====
 
+def _find_mountpoint(path):
+    """Return the filesystem mountpoint that contains path."""
+    r = subprocess.run(
+        ['df', '--output=target', path],
+        capture_output=True, text=True,
+    )
+    lines = [l.strip() for l in r.stdout.strip().splitlines() if l.strip() and l.strip() != 'Mounted on']
+    return lines[0] if lines else '/srv'
+
+
 @app.route('/api/quota', methods=['POST'])
 @require_auth
 def set_quota():
     data = request.json or {}
     user = data.get('user', '')
-    quota_type = data.get('type', 'smb')
     raw_quota = data.get('quota', 0)
-    path = data.get('path', '/srv/samba')
+    path = data.get('path', '/srv')
 
     if not VALID_NAME.match(user):
         return jsonify({'error': 'invalid username'}), 400
@@ -435,17 +444,23 @@ def set_quota():
     except (ValueError, TypeError):
         return jsonify({'error': 'quota must be a positive integer (GB)'}), 400
 
-    if quota_type == 'smb':
-        mount = '/srv/samba'
-    else:
-        try:
-            mount = validate_path(path)
-        except ValueError as e:
-            return jsonify({'error': str(e)}), 400
+    try:
+        safe_path = validate_path(path)
+    except ValueError as e:
+        return jsonify({'error': str(e)}), 400
 
+    mount = _find_mountpoint(safe_path)
     soft_kb = str(quota_gb * 1024 * 1024)
     hard_kb = str((quota_gb + 1) * 1024 * 1024)
-    subprocess.run(['setquota', '-u', user, soft_kb, hard_kb, '0', '0', mount], check=True)
+    try:
+        r = subprocess.run(
+            ['setquota', '-u', user, soft_kb, hard_kb, '0', '0', mount],
+            capture_output=True, text=True,
+        )
+        if r.returncode != 0:
+            return jsonify({'error': r.stderr.strip() or 'setquota failed'}), 500
+    except FileNotFoundError:
+        return jsonify({'error': 'setquota not found — install quota package'}), 500
     return jsonify({'status': 'ok'})