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.

mywebdav/billing/invoice_generator.py

@@ -172,7 +172,7 @@ The MyWebdav Team
 """
         queue_email(
             to_email=invoice.user.email,
-            subject=f"Your RBox Invoice {invoice.invoice_number}",
+            subject=f"Your MyWebdav Invoice {invoice.invoice_number}",
             body=body,
             html=html
         )