Remove native folder picker, use text input instead

Native OS dialog (zenity/AppleScript/PowerShell) fails on Linux with
"native folder picker is not supported on this platform". Replaced:
- dashboard: removed the "+" button, users type mount path manually
- settings: replaced "Add source folder" native dialog with inline
  text input field

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

web/templates/dashboard.html

@@ -5,7 +5,6 @@
   <div class="panel-body">
     <div class="path-input-row">
       <input class="form-input" type="text" id="mountPath" placeholder="/Volumes/JUKEBOX or E:\\">
-      <button type="button" class="button-primary" onclick="pickMountPath()">+</button>
       <button type="button" class="button-secondary" onclick="refreshSelectedDisk()">Refresh</button>
     </div>
     <div class="form-hint">Choose the directory where the removable disk is mounted. The app works with one selected disk at a time in standalone mode.</div>
@@ -290,27 +289,6 @@ function startTaskPoll(taskID) {
   pollTask(taskID);
 }
 
-async function pickFolder() {
-  const response = await fetch('/api/system/pick-folder', { method: 'POST' });
-  const payload = await response.json();
-  if (!response.ok) {
-    throw new Error(payload.error || 'Failed to choose folder');
-  }
-  return payload.path || '';
-}
-
-async function pickMountPath() {
-  try {
-    const path = await pickFolder();
-    if (!path) return;
-    document.getElementById('mountPath').value = path;
-    localStorage.setItem('jukebox.selectedMountPath', path);
-    await refreshSelectedDisk();
-  } catch (error) {
-    toast(error.message || 'Failed to choose folder', 'error');
-  }
-}
-
 async function refreshSelectedDisk() {
   const mountPath = document.getElementById('mountPath').value.trim();
   if (!mountPath) {