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>

electrobun.config.ts

@@ -10,6 +10,9 @@ export default {
 	build: {
 		bun: {
 			entrypoint: "src/bun/index.ts",
+			// ssh2's optional native accelerators must never be bundled; its
+			// runtime try/catch falls back to pure JS when the require fails.
+			external: ["cpu-features", "*.node"],
 		},
 		copy: {
 			"ui/dist/ui/browser": "views/app",