gui: embed compressed dist.zip in the binary for smaller, reproducible builds

Previously `make fetch-gui` extracted the GUI release into cmd/gui/dist/
and the unpacked tree was embedded uncompressed via `//go:embed dist`.

This commits and embeds the GUI bundle (dist.zip) and its release tag
(dist.tag) to the repo so:

- the rclone binary is smaller
- `go build` works on a fresh clone without first running fetch-gui
- a given commit pins an exact GUI version

The "Fetch GUI" step was removed from .github/workflows/build.yml.

.github/workflows/build.yml

@@ -164,11 +164,6 @@ jobs:
           printf "\n\nSystem environment:\n\n"
           env
 
-      - name: Fetch GUI
-        run: make fetch-gui
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-
       - name: Build rclone
         run: |
           make