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/dictation/dictation-stream-sender.ts

@@ -110,7 +110,7 @@ export class DictationStreamSender {
     let sent = 0;
     while (this.sendSeq < this.segments.length) {
       const seq = this.sendSeq;
-      const audio = this.segments[seq]!;
+      const audio = this.segments[seq];
       client.sendDictationStreamChunk(dictationId, seq, audio, this.format);
       this.sendSeq = seq + 1;
       sent += 1;