rename: mCables → CableGUI (project + repo + image + paths)

Full project rename per m's call. Single atomic commit because the
codebase rename is a coupled change — go module path, env vars, DB
default, Docker artefact names, and on-disk mDock paths all flip
together.

- go.mod: module mgit.msbls.de/m/mcables → mgit.msbls.de/m/cablegui
- cmd/mcables → cmd/cablegui (git mv)
- All Go imports rewritten to the new module path
- Env vars: MCABLES_ADDR/MCABLES_DB → CABLEGUI_ADDR/CABLEGUI_DB
- DB default path: data/mcables.db → data/cablegui.db
- Dockerfile + docker-compose.yml: image, container_name, env vars,
  bind-mount /home/m/stacks/mcables → /home/m/stacks/cablegui,
  secrets /home/m/secrets/mcables → /home/m/secrets/cablegui
- Makefile: bin target + run/build commands point at cmd/cablegui
- .gitignore + .dockerignore: /mcables → /cablegui
- README, docs/design.md, CLAUDE.md: prose + paths + image name
- web/static/index.html: <title> + brand
- web/static/main.js + web/web.go: header comment
- internal/exporter: Scene.Source "mcables" → "cablegui"
- internal/server/export.go: error-detail secrets path
- internal/db/migrations/*.sql: header comments (mCables vN → CableGUI vN)

Memory group_id kept as "mcables" to preserve existing memory continuity.
Documented as historical in CLAUDE.md.

go build ./... clean; go test -race ./... green

.dockerignore

@@ -15,7 +15,7 @@ data
 
 # Build artefacts
 bin
-/mcables
+/cablegui
 
 # Editor cruft
 .vscode