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/rpc/aicompose_webpage.go

@@ -43,7 +43,7 @@ func (r *Router) resolveAIComposeStyleWebPage(ctx context.Context, rawURL string
 		Hash:               aiComposeToneWebPageHash(tone),
 		Date:               int(now.Unix()),
 		Type:               aiComposeToneWebPageType,
-		SiteName:           branding.ProductName,
+		SiteName:           branding.ProductName(),
 		Title:              tone.Title,
 		Description:        tone.Prompt,
 		ComposeToneEmojiID: tone.EmojiID,