Remove unnecessary type assertions across codebase

Add oxlint-tsgolint and configure typescript/no-unnecessary-type-assertion
to flag redundant `!` and `as Foo` casts. Type-aware mode is left off by
default to keep `npm run lint` fast; the rule sits configured for when we
turn type-aware on intentionally. Auto-fix removed ~283 redundant casts;
two manual touch-ups: a real tsgolint false positive in split-container.tsx
and a stale ChildProcess import after a double-cast collapsed.

packages/app/src/utils/os-notifications.ts

@@ -132,7 +132,7 @@ function dispatchWebNotificationClick(detail: WebNotificationClickDetail): boole
       cancelable: true,
     },
   );
-  return dispatch(event) === false;
+  return !dispatch(event);
 }
 
 function fallbackNavigateToNotificationTarget(data: Record<string, unknown> | undefined): void {