Refactor agent architecture and add session storage

Major architectural improvements:
- Renamed AgentSession → Agent (state/ → agent/)
- Removed id field from AgentState
- Fixed transport abstraction to pass messages directly instead of using callbacks
- Eliminated circular dependencies in transport creation

Transport changes:
- Changed signature: run(messages, userMessage, config, signal)
- Removed getMessages callback from ProviderTransport and AppTransport
- Transports now filter attachments internally

Session storage:
- Added SessionRepository with IndexedDB backend
- Auto-save sessions after first exchange
- Auto-generate titles from first user message
- Session list dialog with search and delete
- Persistent storage permission dialog
- Browser extension now auto-loads last session

UI improvements:
- ChatPanel creates single AgentInterface instance in setAgent()
- Added drag & drop file upload to MessageEditor
- Fixed artifacts panel auto-opening on session load
- Added "Drop files here" i18n strings
- Changed "Continue Without Saving" → "Continue Anyway"

Web example:
- Complete rewrite of main.ts with clean architecture
- Added check script to package.json
- Session management with URL state
- Editable session titles

Browser extension:
- Added full session storage support
- History and new session buttons
- Auto-load most recent session on open
- Session titles in header

packages/web-ui/src/agent/transports/ProviderTransport.ts

@@ -0,0 +1,56 @@
+import { type AgentContext, agentLoop, type Message, type PromptConfig, type UserMessage } from "@mariozechner/pi-ai";
+import { getAppStorage } from "../../storage/app-storage.js";
+import type { AgentRunConfig, AgentTransport } from "./types.js";
+
+/**
+ * Transport that calls LLM providers directly.
+ * Optionally routes calls through a CORS proxy if enabled in settings.
+ */
+export class ProviderTransport implements AgentTransport {
+	async *run(messages: Message[], userMessage: Message, cfg: AgentRunConfig, signal?: AbortSignal) {
+		// Get API key from storage
+		const apiKey = await getAppStorage().providerKeys.getKey(cfg.model.provider);
+		if (!apiKey) {
+			throw new Error("no-api-key");
+		}
+
+		// Check if CORS proxy is enabled
+		const proxyEnabled = await getAppStorage().settings.get<boolean>("proxy.enabled");
+		const proxyUrl = await getAppStorage().settings.get<string>("proxy.url");
+
+		// Clone model and modify baseUrl if proxy is enabled
+		let model = cfg.model;
+		if (proxyEnabled && proxyUrl && cfg.model.baseUrl) {
+			model = {
+				...cfg.model,
+				baseUrl: `${proxyUrl}/?url=${encodeURIComponent(cfg.model.baseUrl)}`,
+			};
+		}
+
+		// Filter out attachments from messages
+		const filteredMessages = messages.map((m) => {
+			if (m.role === "user") {
+				const { attachments, ...rest } = m as any;
+				return rest;
+			}
+			return m;
+		});
+
+		const context: AgentContext = {
+			systemPrompt: cfg.systemPrompt,
+			messages: filteredMessages,
+			tools: cfg.tools,
+		};
+
+		const pc: PromptConfig = {
+			model,
+			reasoning: cfg.reasoning,
+			apiKey,
+		};
+
+		// Yield events from agentLoop
+		for await (const ev of agentLoop(userMessage as unknown as UserMessage, context, pc, signal)) {
+			yield ev;
+		}
+	}
+}