feat(error): add error handling foundation (Day 0)

- extractErrorMessage(err: unknown) normalizes any error shape to string
- handleError/handleInfraError dual utilities (user-facing vs infra-only)
- error-classifier extended with ipc/database/filesystem types (9 total)
- Toast rate-limiting (max 3 per type per 30s) in notifications store
- Infrastructure bridges use documented console.warn (recursion prevention)
- 13 new tests for extractErrorMessage

src/lib/utils/extract-error-message.ts

@@ -0,0 +1,26 @@
+// Extract a human-readable message from any error shape
+
+/**
+ * Normalize any caught error value into a readable string.
+ * Handles: string, Error, Tauri IPC errors, Rust AppError {kind, detail}, objects with .message.
+ */
+export function extractErrorMessage(err: unknown): string {
+  if (typeof err === 'string') return err;
+  if (err instanceof Error) return err.message;
+  if (err && typeof err === 'object') {
+    const obj = err as Record<string, unknown>;
+    // Rust AppError tagged enum: { kind: "Database", detail: { ... } }
+    if ('kind' in obj && typeof obj.kind === 'string') {
+      if ('detail' in obj && obj.detail) {
+        const detail = typeof obj.detail === 'string'
+          ? obj.detail
+          : JSON.stringify(obj.detail);
+        return `${obj.kind}: ${detail}`;
+      }
+      return obj.kind;
+    }
+    // Generic object with message property
+    if ('message' in obj && typeof obj.message === 'string') return obj.message;
+  }
+  return String(err);
+}