Refactor to Store-based architecture

- Create base Store class with private backend and protected getBackend()
- Add SettingsStore, ProviderKeysStore, SessionsStore
- Each store defines its own schema via getConfig()
- AppStorage now takes stores + backend in constructor
- Remove SessionsRepository (logic moved to SessionsStore)
- Update all consumers to use store API (storage.settings.get/set, storage.providerKeys.get/set)
- Update example app to follow new pattern: create stores, gather configs, create backend, wire
- Benefits: stores own their schema, no circular deps, cleaner separation

packages/web-ui/src/storage/store.ts

@@ -0,0 +1,33 @@
+import type { StorageBackend, StoreConfig } from "./types.js";
+
+/**
+ * Base class for all storage stores.
+ * Each store defines its IndexedDB schema and provides domain-specific methods.
+ */
+export abstract class Store {
+	private backend: StorageBackend | null = null;
+
+	/**
+	 * Returns the IndexedDB configuration for this store.
+	 * Defines store name, key path, and indices.
+	 */
+	abstract getConfig(): StoreConfig;
+
+	/**
+	 * Sets the storage backend. Called by AppStorage after backend creation.
+	 */
+	setBackend(backend: StorageBackend): void {
+		this.backend = backend;
+	}
+
+	/**
+	 * Gets the storage backend. Throws if backend not set.
+	 * Concrete stores must use this to access the backend.
+	 */
+	protected getBackend(): StorageBackend {
+		if (!this.backend) {
+			throw new Error(`Backend not set on ${this.constructor.name}`);
+		}
+		return this.backend;
+	}
+}