feat(web): Integrate M4-LLM error handling improvements

Port high-value features from work/m4-llm branch into develop's
security-hardened codebase:

- Separate LLM vs persistence error handling in useChat (shows
  assistant response even when save fails)
- Add structured error context logging with errorType, messagePreview,
  messageCount fields for debugging
- Enforce state invariant in useChatOverlay: cannot be minimized when
  closed
- Add onStorageError callback with user-friendly messages and
  per-error-type deduplication
- Add error logging to Chat imperative handle methods
- Create Chat.test.tsx with loadConversation failure mode tests

Skipped from work/m4-llm (superseded by develop):
- AbortSignal timeout (develop has centralized client timeout)
- Custom toast system (duplicates @mosaic/ui)
- ErrorBoundary (develop has its own)
- WebSocket typed events (develop's ref-based pattern is superior)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/utils/safe-json.test.ts

@@ -170,7 +170,12 @@ describe("isMessageArray", () => {
 describe("isChatOverlayState", () => {
   it("should return true for a valid ChatOverlayState", () => {
     expect(isChatOverlayState({ isOpen: true, isMinimized: false })).toBe(true);
-    expect(isChatOverlayState({ isOpen: false, isMinimized: true })).toBe(true);
+    expect(isChatOverlayState({ isOpen: true, isMinimized: true })).toBe(true);
+    expect(isChatOverlayState({ isOpen: false, isMinimized: false })).toBe(true);
+  });
+
+  it("should reject invalid state: closed AND minimized (invariant violation)", () => {
+    expect(isChatOverlayState({ isOpen: false, isMinimized: true })).toBe(false);
   });
 
   it("should return false for non-object values", () => {

apps/web/src/lib/utils/safe-json.ts

@@ -76,12 +76,16 @@ export function isMessageArray(value: unknown): value is {
 /**
  * Type guard: validates ChatOverlayState shape
  * Expects { isOpen: boolean, isMinimized: boolean }
+ * Enforces invariant: cannot be minimized when closed
  */
 export function isChatOverlayState(
   value: unknown
 ): value is { isOpen: boolean; isMinimized: boolean } {
   if (!isRecord(value)) return false;
-  return typeof value.isOpen === "boolean" && typeof value.isMinimized === "boolean";
+  if (typeof value.isOpen !== "boolean" || typeof value.isMinimized !== "boolean") return false;
+  // Invariant: cannot be minimized when closed
+  if (!value.isOpen && value.isMinimized) return false;
+  return true;
 }
 
 /**