fix: remove last-provider deletion restriction for OMO/OMO Slim plugins

OMO and OMO Slim are OpenCode plugins, not standalone apps — users
should be able to fully remove them. Remove the count-based guard that
prevented deleting the last active provider, and clean up the now-unused
provider-count API surface across the full stack.

src/lib/api/omo.ts

@@ -5,7 +5,6 @@ export const omoApi = {
   readLocalFile: (): Promise<OmoLocalFileData> => invoke("read_omo_local_file"),
   getCurrentOmoProviderId: (): Promise<string> =>
     invoke("get_current_omo_provider_id"),
-  getOmoProviderCount: (): Promise<number> => invoke("get_omo_provider_count"),
   disableCurrentOmo: (): Promise<void> => invoke("disable_current_omo"),
 };
 
@@ -14,7 +13,5 @@ export const omoSlimApi = {
     invoke("read_omo_slim_local_file"),
   getCurrentProviderId: (): Promise<string> =>
     invoke("get_current_omo_slim_provider_id"),
-  getProviderCount: (): Promise<number> =>
-    invoke("get_omo_slim_provider_count"),
   disableCurrent: (): Promise<void> => invoke("disable_current_omo_slim"),
 };