fix(web): harden typed SPA chat lifecycle

Co-Authored-By: Claude Haiku 4.5 <[email protected]>

apps/web/src/spa/chat/runtime-guards.ts

@@ -0,0 +1,41 @@
+/**
+ * Socket.IO payloads are only statically typed at the call site — a
+ * misbehaving or compromised gateway can send anything at runtime. These
+ * guards protect the dereference sites that would otherwise throw (`.map` on
+ * a non-array, `.toFixed` on a non-number) or render an object as a React
+ * child.
+ */
+
+export function asString(value: unknown, fallback = ''): string {
+  return typeof value === 'string' ? value : fallback;
+}
+
+export function asFiniteNumber(value: unknown, fallback = 0): number {
+  return typeof value === 'number' && Number.isFinite(value) ? value : fallback;
+}
+
+/** Like `asFiniteNumber`, but returns `null` on failure instead of a numeric
+ * fallback — callers that must not fabricate a plausible-looking value (e.g.
+ * `0 tokens` / `$0.0000` for genuinely unknown usage) use this to render an
+ * honest "unavailable" label instead. */
+export function asFiniteNumberOrNull(value: unknown): number | null {
+  return typeof value === 'number' && Number.isFinite(value) ? value : null;
+}
+
+export function asStringArray(value: unknown): string[] {
+  return Array.isArray(value) && value.every((item) => typeof item === 'string') ? value : [];
+}
+
+export function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null;
+}
+
+/** The single point of truth for what counts as a valid conversation ID
+ * anywhere a scoped server event may adopt one into state — a non-empty
+ * string, nothing else. Every site that establishes or compares
+ * `state.conversationId` against a raw socket payload must route through
+ * this guard so a malformed first frame (null/object/number/empty string)
+ * can never be adopted verbatim. */
+export function asConversationId(value: unknown): string | null {
+  return typeof value === 'string' && value.length > 0 ? value : null;
+}