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>

src/bun/store/connections.ts

@@ -0,0 +1,70 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+import type { ConnFolder, ConnectionsFile, SavedConnection } from "../../shared/types";
+import { appDataDir } from "./paths";
+
+const EMPTY: ConnectionsFile = { version: 1, folders: [], connections: [] };
+
+/**
+ * Plain-JSON store for saved connections (passwords included — the user's
+ * explicit v1 choice). Kept at 0600 and written atomically via tmp+rename.
+ */
+export class ConnectionStore {
+	private readonly file = path.join(appDataDir(), "connections.json");
+	private data: ConnectionsFile | null = null;
+
+	async load(): Promise<ConnectionsFile> {
+		if (!this.data) {
+			try {
+				this.data = JSON.parse(await fs.readFile(this.file, "utf8")) as ConnectionsFile;
+			} catch {
+				this.data = structuredClone(EMPTY);
+			}
+		}
+		return this.data;
+	}
+
+	async upsertConnection(connection: SavedConnection): Promise<string> {
+		const data = await this.load();
+		const saved = { ...connection, id: connection.id || crypto.randomUUID() };
+		const index = data.connections.findIndex((c) => c.id === saved.id);
+		if (index >= 0) data.connections[index] = saved;
+		else data.connections.push(saved);
+		await this.save();
+		return saved.id;
+	}
+
+	async deleteConnection(id: string): Promise<void> {
+		const data = await this.load();
+		data.connections = data.connections.filter((c) => c.id !== id);
+		await this.save();
+	}
+
+	async upsertFolder(folder: ConnFolder): Promise<string> {
+		const data = await this.load();
+		const saved = { ...folder, id: folder.id || crypto.randomUUID() };
+		const index = data.folders.findIndex((f) => f.id === saved.id);
+		if (index >= 0) data.folders[index] = saved;
+		else data.folders.push(saved);
+		await this.save();
+		return saved.id;
+	}
+
+	/** Deleting a folder moves its connections to the root. */
+	async deleteFolder(id: string): Promise<void> {
+		const data = await this.load();
+		data.folders = data.folders.filter((f) => f.id !== id);
+		for (const connection of data.connections) {
+			if (connection.folderId === id) connection.folderId = null;
+		}
+		await this.save();
+	}
+
+	private async save(): Promise<void> {
+		await fs.mkdir(path.dirname(this.file), { recursive: true });
+		const tmp = `${this.file}.tmp`;
+		await fs.writeFile(tmp, JSON.stringify(this.data, null, "\t"), { mode: 0o600 });
+		await fs.rename(tmp, this.file);
+		await fs.chmod(this.file, 0o600);
+	}
+}