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/app/passkey/service.go

@@ -71,7 +71,7 @@ func NewService(creds store.PasskeyStore, challenges store.PasskeyChallengeStore
 		creds:        creds,
 		challenges:   challenges,
 		rpID:         rpID,
-		rpName:       branding.ProductName,
+		rpName:       branding.ProductName(),
 		dcID:         dcID,
 		challengeTTL: defaultChallengeTTL,
 		now:          time.Now,