chore: rebrand all RBox references to MyWebdav across codebase and docs

Replace every occurrence of "RBox" with "MyWebdav" in source files, configuration,
legal documents, UI components, and test fixtures. This includes renaming the
`RBoxApp` class to `MyWebdavApp` in the JavaScript component, updating the
TOTP issuer from "RBox" to "MyWebdav" in the auth router, changing the
application description in the argument parser, and updating all user-facing
strings in HTML templates, legal policies, and the manifest file. The
`.env.example` default for `TOTP_ISSUER` is also updated accordingly.

Makefile

@@ -11,7 +11,7 @@ all:
 	$(MYWEBDAV) --port 9004
 
 help:
-	@echo "RBox Development Makefile"
+	@echo "MyWebdav Development Makefile"
 	@echo ""
 	@echo "Available commands:"
 	@echo "  make all             Run the application on port 9004"
@@ -44,7 +44,7 @@ dev:
 	@echo "Development dependencies installed successfully"
 
 run:
-	@echo "Starting RBox application..."
+	@echo "Starting MyWebdav application..."
 	@echo "Access the application at http://localhost:8000"
 	$(PYTHON) -m mywebdav.main