feat(errors): add Sentry observability behind a sinks facade

logError / reportError now route through a pair of sinks installed at
bootstrap by each process (main + every renderer entry). No call site
knows about Sentry — if sentryDsn is empty, the sinks simply aren't
installed and logError/reportError stay console-only.

- appConfig.sentryDsn: per-env string (empty for now — populate when
  ops creates the DSNs). Empty is the no-op mode for dev.
- lib/errors.ts: installErrorSinks({ capture, breadcrumb }) gates
  Sentry.captureException / Sentry.addBreadcrumb. Everything flows
  through toUserMessage and the two existing call types.
- lib/sentry.ts: initSentryRenderer() for the main window + autoplay,
  huddle, and screen-record renderers.
- main/sentry.ts: initSentryMain() runs before anything else in
  main.ts to catch bootstrap failures. Captures uncaught exceptions
  and the crash reporter automatically.
- main/ipc-utils.ts::safeHandle now routes through reportError.
- main.ts::fetchLinkMetadata logs via logError.

js/src/lib/errors.ts

@@ -59,17 +59,34 @@ export function toUserMessage(err: unknown): string {
 }
 
 type ErrorContext = Record<string, unknown>;
+type ErrorSink = (err: unknown, context?: ErrorContext) => void;
 
-/** Expected-but-recordable failures. Dev logs; prod is currently a no-op. */
+// Sentry (or any observability backend) installs itself via `installErrorSinks`
+// from renderer.tsx / main.ts. Until then, logError is a dev-only console call
+// and reportError always prints — no call site needs to know.
+let captureSink: ErrorSink | null = null;
+let breadcrumbSink: ErrorSink | null = null;
+
+export function installErrorSinks(sinks: {
+  capture: ErrorSink;
+  breadcrumb: ErrorSink;
+}): void {
+  captureSink = sinks.capture;
+  breadcrumbSink = sinks.breadcrumb;
+}
+
+/** Expected-but-recordable failures. Breadcrumb only — never pages anyone. */
 export function logError(err: unknown, context?: ErrorContext): void {
   if (appEnv === "dev") {
     // eslint-disable-next-line no-console
     console.error("[error]", err, context ?? {});
   }
+  breadcrumbSink?.(err, context);
 }
 
-/** Unexpected failures the user may not see. Always surfaces somewhere. */
+/** Unexpected failures the user may not see. Always captured. */
 export function reportError(err: unknown, context?: ErrorContext): void {
   // eslint-disable-next-line no-console
   console.error("[error]", err, context ?? {});
+  captureSink?.(err, context);
 }