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/app.html

@@ -2,6 +2,16 @@
   <header class="flex items-center gap-2 border-b border-border px-3 py-1.5">
     <ng-icon name="mynaArrowLeftRight" class="text-muted-foreground" size="0.9rem" />
     <h1 class="text-[13px] font-semibold tracking-tight">{{ appName }}</h1>
+    <div class="flex-1"></div>
+    @if (rpc.available) {
+      <button
+        (click)="dialogs.manager.set(true)"
+        title="Manage connections"
+        class="grid size-6 place-items-center rounded-md text-muted-foreground hover:bg-accent hover:text-foreground"
+      >
+        <ng-icon name="mynaCog" size="0.95rem" />
+      </button>
+    }
   </header>
 
   @if (!rpc.available) {
@@ -70,6 +80,9 @@
   }
 
   <!-- dialogs -->
+  @if (dialogs.manager()) {
+    <app-connection-manager />
+  }
   @if (dialogs.prompt(); as req) {
     <app-prompt-dialog [request]="req" />
   }