fix(build): extract STORED_LOGO_SENTINEL to non-server module

Vite/React Router refused to bundle the client because
app/routes/app.settings.tsx imported the constant from a .server file
and used it inside the route component (not just loader/action), so it
could not be tree-shaken out.

Move the sentinel to logoCache.constants.ts, re-export from
logoCache.server.ts for backwards compatibility, and import the constant
from constants in the route while keeping the server-only functions
(deleteStoredLogo, storeUploadedLogo) imported from .server (they are
only referenced inside the action and get tree-shaken correctly).

app/services/invoice/logoCache.constants.ts

@@ -0,0 +1,6 @@
+/**
+ * Shared (client + server safe) constants for the logo cache.
+ * Kept in a non-`.server` module so route components can import them
+ * without React Router pulling server-only code into the client bundle.
+ */
+export const STORED_LOGO_SENTINEL = "stored://shop-logo";