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/voice-runtime.ts

@@ -295,7 +295,7 @@ export function createVoiceRuntime(deps: VoiceRuntimeDeps): VoiceRuntime {
 
   function activateNextPlaybackGroup(): void {
     while (playback.orderedGroupIds.length > 0) {
-      const groupId = playback.orderedGroupIds[0]!;
+      const groupId = playback.orderedGroupIds[0];
       if (playback.groups.has(groupId)) {
         playback.activeGroupId = groupId;
         return;