fix: make RDP drive redirection writable on the stock deployment (#1333)

* fix: make RDP drive redirection writable on the stock deployment

The default drive-path was /drive on the guacd side, which the official
guacd image cannot create as its non-root user, so every upload was
refused with guacd's raw "FAIL (CANNOT OPEN)" ack. Default to
GUACD_DRIVE_PATH (set to the shared termix-data volume in compose) with
one folder per user, and explain guacd's refusal in the file browser.

* style: format RDP drive settings

src/backend/tests/hosts/guacamole/drive-settings.test.ts

@@ -0,0 +1,34 @@
+import { describe, expect, it } from "vitest";
+import { withDriveSettings } from "../../../hosts/guacamole/drive-settings.js";
+
+describe("withDriveSettings", () => {
+  it("gives each user a folder under GUACD_DRIVE_PATH and creates it", () => {
+    expect(
+      withDriveSettings({ "enable-drive": true }, "user-1", {
+        GUACD_DRIVE_PATH: "/termix-data/rdp-drive/",
+      }),
+    ).toEqual({
+      "enable-drive": true,
+      "drive-path": "/termix-data/rdp-drive/user-1",
+      "create-drive-path": true,
+    });
+  });
+
+  it("falls back to /drive when the environment says nothing", () => {
+    expect(
+      withDriveSettings({ "enable-drive": true }, "user-1", {}),
+    ).toMatchObject({ "drive-path": "/drive/user-1" });
+  });
+
+  it("leaves a host-chosen drive-path alone", () => {
+    const config = { "enable-drive": true, "drive-path": "/mnt/share" };
+    expect(
+      withDriveSettings(config, "user-1", { GUACD_DRIVE_PATH: "/x" }),
+    ).toBe(config);
+  });
+
+  it("does nothing when the drive is not enabled", () => {
+    const config = { "enable-drive": false };
+    expect(withDriveSettings(config, "user-1")).toBe(config);
+  });
+});