branding: make product identity runtime-configurable

Adopt upstream owpengram/owpengram-server's branding.Config/Configure/
Current in place of the old package-level constants. This is the piece the
earlier merge attempt was blocked on (internal/branding failed to import
during that merge). The default identity is unchanged -- every existing
ProductName/ProductUsername/... default still reads "OwpenGram" -- so this
is a pure capability add: nothing currently calls Configure, and every call
site now reads the current snapshot via a function instead of a compile-time
constant.

Five string templates that concatenated branding.ProductName into a `const`
had to become `var`, since a func call is no longer a valid const operand.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/domain/system.go

@@ -172,8 +172,8 @@ func OfficialSystemUser() User {
 		ID:         OfficialSystemUserID,
 		AccessHash: 6599886787491911851,
 		Phone:      "42777",
-		FirstName:  branding.ProductName,
-		Username:   branding.ProductUsername,
+		FirstName:  branding.ProductName(),
+		Username:   branding.ProductUsername(),
 		Verified:   true,
 		Support:    true,
 	}