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/langpack/service.go

@@ -54,7 +54,7 @@ func newServiceWithCacheLimits(packs store.LangPackStore, maxBytes int64, maxEnt
 		packs:         packs,
 		packCache:     newLangPackCache(maxBytes, maxEntries),
 		languageCache: newLanguageListCache(languageEntries),
-		publicBaseURL: branding.DefaultPublicURL,
+		publicBaseURL: branding.PublicBaseURL(),
 	}
 }