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/components/use-web-scrollbar.tsx

@@ -140,9 +140,9 @@ export function useWebScrollViewScrollbar(
       const scrollable = scrollableRef.current;
       if (!scrollable) return;
       if ("scrollToOffset" in scrollable) {
-        (scrollable as FlatList).scrollToOffset({ offset, animated: false });
+        scrollable.scrollToOffset({ offset, animated: false });
       } else {
-        (scrollable as ScrollView).scrollTo({ y: offset, animated: false });
+        scrollable.scrollTo({ y: offset, animated: false });
       }
     },
     [scrollableRef],