Add Anthropic prompt caching, pluggable storage, and CORS proxy support

Storage Architecture:
- New pluggable storage system with backends (LocalStorage, ChromeStorage, IndexedDB)
- SettingsRepository for app settings (proxy config, etc.)
- ProviderKeysRepository for API key management
- AppStorage with global accessors (getAppStorage, setAppStorage, initAppStorage)

Transport Refactoring:
- Renamed DirectTransport → ProviderTransport (calls LLM providers with optional CORS proxy)
- Renamed ProxyTransport → AppTransport (uses app server with user auth)
- Updated TransportMode: "direct" → "provider", "proxy" → "app"

CORS Proxy Integration:
- ProviderTransport checks proxy.enabled/proxy.url from storage
- When enabled, modifies model baseUrl to route through proxy: {proxyUrl}/?url={originalBaseUrl}
- ProviderKeyInput test function also honors proxy settings
- Settings dialog with Proxy tab (Switch toggle, URL input, explanatory description)

Anthropic Prompt Caching:
- System prompt cached with cache_control markers (both OAuth and regular API keys)
- Last user message cached to cache conversation history
- Saves 90% on input tokens for cached content (10x cost reduction)

Settings Dialog Improvements:
- Configurable tab system with SettingsTab base class
- ApiKeysTab and ProxyTab as custom elements
- Switch toggle for proxy enable (instead of Checkbox)
- Explanatory paragraphs for each tab
- ApiKeyPromptDialog reuses ProviderKeyInput component

Removed:
- Deprecated ApiKeysDialog (replaced by ProviderKeyInput in SettingsDialog)
- Old storage-adapter and key-store (replaced by new storage architecture)

packages/web-ui/src/dialogs/ApiKeyPromptDialog.ts

@@ -0,0 +1,76 @@
+import { DialogBase, DialogContent, DialogHeader, html } from "@mariozechner/mini-lit";
+import { customElement, state } from "lit/decorators.js";
+import "../components/ProviderKeyInput.js";
+import { getAppStorage } from "../storage/app-storage.js";
+import { i18n } from "../utils/i18n.js";
+
+@customElement("api-key-prompt-dialog")
+export class ApiKeyPromptDialog extends DialogBase {
+	@state() private provider = "";
+
+	private resolvePromise?: (success: boolean) => void;
+	private unsubscribe?: () => void;
+
+	protected modalWidth = "min(500px, 90vw)";
+	protected modalHeight = "auto";
+
+	static async prompt(provider: string): Promise<boolean> {
+		const dialog = new ApiKeyPromptDialog();
+		dialog.provider = provider;
+		dialog.open();
+
+		return new Promise((resolve) => {
+			dialog.resolvePromise = resolve;
+		});
+	}
+
+	override async connectedCallback() {
+		super.connectedCallback();
+
+		// Poll for key existence - when key is added, resolve and close
+		const checkInterval = setInterval(async () => {
+			const hasKey = await getAppStorage().providerKeys.hasKey(this.provider);
+			if (hasKey) {
+				clearInterval(checkInterval);
+				if (this.resolvePromise) {
+					this.resolvePromise(true);
+					this.resolvePromise = undefined;
+				}
+				this.close();
+			}
+		}, 500);
+
+		this.unsubscribe = () => clearInterval(checkInterval);
+	}
+
+	override disconnectedCallback() {
+		super.disconnectedCallback();
+		if (this.unsubscribe) {
+			this.unsubscribe();
+			this.unsubscribe = undefined;
+		}
+	}
+
+	override close() {
+		super.close();
+		if (this.resolvePromise) {
+			this.resolvePromise(false);
+		}
+	}
+
+	protected override renderContent() {
+		return html`
+			${DialogContent({
+				children: html`
+					${DialogHeader({
+						title: i18n("API Key Required"),
+						description: i18n("Enter your API key for {provider}").replace("{provider}", this.provider),
+					})}
+					<div class="mt-4">
+						<provider-key-input .provider=${this.provider}></provider-key-input>
+					</div>
+				`,
+			})}
+		`;
+	}
+}