Promote Files into shared canvas surface

Add Files to the universal canvas rail and sidebar flow, with a reusable floating surface modal chrome that matches Browser/Desktop behavior.

Make the File Browser modal draggable/resizable with Focus mode, keep Editor on the same draggable modal helper, and preserve dock/undock handoff state.

Harden File Browser startup so empty paths resolve to the default workdir, restyle the Up control, compact New file/New folder actions, and hide Modified before Name/Size in narrow containers.

Update DOX contracts and focused regression coverage for the new Files surface, modal chrome, default-path fallback, and compact layout behavior.

api/get_work_dir_files.py

@@ -9,7 +9,7 @@ class GetWorkDirFiles(ApiHandler):
         return ["GET"]
 
     async def process(self, input: dict, request: Request) -> dict | Response:
-        current_path = request.args.get("path", "")
+        current_path = request.args.get("path", "") or "$WORK_DIR"
         if current_path == "$WORK_DIR":
             # if runtime.is_development():
             #     current_path = "work_dir"

api/get_work_dir_files.py.dox.md

@@ -29,6 +29,7 @@
 ## Key Concepts
 
 - Important called helpers/classes observed in the source: `FileBrowser`, `browser.get_files`, `runtime.call_development_function`.
+- Empty `path` requests and explicit `$WORK_DIR` requests resolve to the default workdir path before `FileBrowser` is called, so the WebUI never receives an empty startup path for the default file browser view.
 - Keep request/response, tool, or helper semantics documented here at the same time as source changes.
 
 ## Work Guidance