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/desktop/daemon/desktop-daemon-transport.ts

@@ -13,7 +13,7 @@ function encodeBinaryToBase64(data: Uint8Array | ArrayBuffer): string {
   const bytes = data instanceof ArrayBuffer ? new Uint8Array(data) : data;
   let binary = "";
   for (let index = 0; index < bytes.length; index += 1) {
-    binary += String.fromCharCode(bytes[index]!);
+    binary += String.fromCharCode(bytes[index]);
   }
   return globalThis.btoa(binary);
 }