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/voice/audio-engine.native.ts

@@ -38,8 +38,8 @@ function resamplePcm16(pcm: Uint8Array, fromRate: number, toRate: number): Uint8
     if (i + 1 >= pcm.length) {
       return 0;
     }
-    const lo = pcm[i]!;
-    const hi = pcm[i + 1]!;
+    const lo = pcm[i];
+    const hi = pcm[i + 1];
     let value = (hi << 8) | lo;
     if (value & 0x8000) {
       value = value - 0x10000;