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/components/browser-pane.electron.tsx

@@ -461,7 +461,7 @@ export function BrowserPane({
     };
     const handleFaviconUpdated = (event: Event) => {
       const favicons = Array.isArray((event as Event & { favicons?: unknown[] }).favicons)
-        ? (((event as Event & { favicons?: string[] }).favicons as string[] | undefined) ?? [])
+        ? ((event as Event & { favicons?: string[] }).favicons ?? [])
         : [];
       updateBrowserRef.current(browserIdRef.current, { faviconUrl: favicons[0] ?? null });
     };