P4: SFTP provider, connection store, connection manager UI

- SftpProvider (ssh2): password + key auth, keepalive, clean-dispose vs
  unexpected-close distinction, POSIX path helpers, s->ms attr normalization
- ConnectionStore: plain JSON at appDataDir, 0600, atomic writes, folders
- connect/disconnect + store CRUD handlers; sessionClosed push -> panels fall
  back to local and in-flight jobs fail
- UI: session dropdown per panel (local/saved connections/manage), disconnect
  button, connection manager modal (folders, edit form, save/connect)
- electrobun bundler: external cpu-features/*.node (ssh2 optional natives)
- RpcService: gate requests on RPC socket open; defer initial panel loads past
  first render
- verified against 192.168.10.122: full op matrix, upload/download/remote-remote
  recursive transfers, roundtrip integrity, recursive remote delete, store CRUD

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/file-panel/file-panel.html

@@ -1,13 +1,85 @@
 <!-- session strip -->
-<div class="flex items-center gap-1.5 border-b border-border px-2 py-1.5">
+<div class="relative flex items-center gap-1.5 border-b border-border px-2 py-1.5">
   <button
-    class="flex items-center gap-1.5 rounded-md px-2 py-1 text-xs font-medium hover:bg-accent"
+    (click)="menuOpen.set(!menuOpen())"
+    class="flex min-w-0 items-center gap-1.5 rounded-md px-2 py-1 text-xs font-medium hover:bg-accent"
     title="Choose what this panel shows"
   >
-    <ng-icon name="mynaHardDrive" class="text-muted-foreground" size="0.9rem" />
-    <span>{{ panel().title() }}</span>
-    <ng-icon name="mynaChevronDown" class="text-muted-foreground" size="0.8rem" />
+    @if (panel().connecting()) {
+      <ng-icon name="mynaSpinner" class="shrink-0 animate-spin text-primary" size="0.9rem" />
+    } @else {
+      <ng-icon
+        [name]="isRemote() ? 'mynaServers' : 'mynaHardDrive'"
+        [class]="isRemote() ? 'shrink-0 text-primary' : 'shrink-0 text-muted-foreground'"
+        size="0.9rem"
+      />
+    }
+    <span class="truncate">{{ panel().title() }}</span>
+    <ng-icon name="mynaChevronDown" class="shrink-0 text-muted-foreground" size="0.8rem" />
   </button>
+  @if (isRemote()) {
+    <button
+      (click)="disconnect()"
+      title="Disconnect (back to local)"
+      class="grid size-6 shrink-0 place-items-center rounded-md text-muted-foreground hover:bg-accent hover:text-foreground"
+    >
+      <ng-icon name="mynaLogout" size="0.85rem" />
+    </button>
+  }
+
+  @if (menuOpen()) {
+    <div class="fixed inset-0 z-20" (click)="menuOpen.set(false)"></div>
+    <div
+      class="absolute top-full left-2 z-30 mt-1 max-h-80 w-64 overflow-y-auto rounded-lg border border-border bg-card py-1 text-card-foreground shadow-xl"
+    >
+      <button
+        (click)="pickLocal()"
+        class="flex w-full items-center gap-2 px-3 py-1.5 text-left text-xs hover:bg-accent"
+      >
+        <ng-icon name="mynaHardDrive" size="0.9rem" class="shrink-0 text-muted-foreground" />
+        <span class="flex-1">Local</span>
+        @if (!isRemote()) {
+          <ng-icon name="mynaCheck" size="0.85rem" class="shrink-0 text-primary" />
+        }
+      </button>
+
+      @if (connections.grouped().root.length || connections.grouped().folders.length) {
+        <div class="my-1 border-t border-border"></div>
+      }
+      @for (connection of connections.grouped().root; track connection.id) {
+        <button
+          (click)="pickConnection(connection)"
+          class="flex w-full items-center gap-2 px-3 py-1.5 text-left text-xs hover:bg-accent"
+        >
+          <ng-icon name="mynaServers" size="0.9rem" class="shrink-0 text-muted-foreground" />
+          <span class="flex-1 truncate">{{ connection.name }}</span>
+        </button>
+      }
+      @for (group of connections.grouped().folders; track group.folder.id) {
+        <div class="px-3 pt-1.5 pb-0.5 text-[10px] font-medium tracking-wide text-muted-foreground uppercase">
+          {{ group.folder.name }}
+        </div>
+        @for (connection of group.connections; track connection.id) {
+          <button
+            (click)="pickConnection(connection)"
+            class="flex w-full items-center gap-2 py-1.5 pr-3 pl-5 text-left text-xs hover:bg-accent"
+          >
+            <ng-icon name="mynaServers" size="0.9rem" class="shrink-0 text-muted-foreground" />
+            <span class="flex-1 truncate">{{ connection.name }}</span>
+          </button>
+        }
+      }
+
+      <div class="my-1 border-t border-border"></div>
+      <button
+        (click)="openManager()"
+        class="flex w-full items-center gap-2 px-3 py-1.5 text-left text-xs hover:bg-accent"
+      >
+        <ng-icon name="mynaCog" size="0.9rem" class="shrink-0 text-muted-foreground" />
+        Manage connections…
+      </button>
+    </div>
+  }
 </div>
 
 <!-- path bar -->