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/attachments/web/indexeddb-attachment-store.test.ts

@@ -22,9 +22,9 @@ class FakeRequest<T = unknown> {
 class FakeObjectStore {
   constructor(private readonly onPut: (record: unknown) => void) {}
 
-  put(record: unknown): FakeRequest<unknown> {
+  put(record: unknown): FakeRequest {
     this.onPut(record);
-    const request = new FakeRequest<unknown>();
+    const request = new FakeRequest();
     queueMicrotask(() => request.emit("success"));
     return request;
   }

packages/app/src/attachments/web/indexeddb-attachment-store.ts

@@ -61,7 +61,7 @@ function runTx<T>(
     const request = run(store);
 
     request.addEventListener("success", () => {
-      resolve(request.result as T);
+      resolve(request.result);
     });
 
     request.addEventListener("error", () => {