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/dialogs/PersistentStorageDialog.ts

@@ -0,0 +1,141 @@
+import { Button, DialogBase, DialogContent, DialogHeader, html } from "@mariozechner/mini-lit";
+import { customElement, state } from "lit/decorators.js";
+import { i18n } from "../utils/i18n.js";
+
+@customElement("persistent-storage-dialog")
+export class PersistentStorageDialog extends DialogBase {
+	@state() private requesting = false;
+
+	private resolvePromise?: (userApproved: boolean) => void;
+
+	protected modalWidth = "min(500px, 90vw)";
+	protected modalHeight = "auto";
+
+	/**
+	 * Request persistent storage permission.
+	 * Returns true if browser granted persistent storage, false otherwise.
+	 */
+	static async request(): Promise<boolean> {
+		// Check if already persisted
+		if (navigator.storage?.persisted) {
+			const alreadyPersisted = await navigator.storage.persisted();
+			if (alreadyPersisted) {
+				console.log("✓ Persistent storage already granted");
+				return true;
+			}
+		}
+
+		// Show dialog and wait for user response
+		const dialog = new PersistentStorageDialog();
+		dialog.open();
+
+		const userApproved = await new Promise<boolean>((resolve) => {
+			dialog.resolvePromise = resolve;
+		});
+
+		if (!userApproved) {
+			console.warn("⚠ User declined persistent storage - sessions may be lost");
+			return false;
+		}
+
+		// User approved, request from browser
+		if (!navigator.storage?.persist) {
+			console.warn("⚠ Persistent storage API not available");
+			return false;
+		}
+
+		try {
+			const granted = await navigator.storage.persist();
+			if (granted) {
+				console.log("✓ Persistent storage granted - sessions will be preserved");
+			} else {
+				console.warn("⚠ Browser denied persistent storage - sessions may be lost under storage pressure");
+			}
+			return granted;
+		} catch (error) {
+			console.error("Failed to request persistent storage:", error);
+			return false;
+		}
+	}
+
+	private handleGrant() {
+		if (this.resolvePromise) {
+			this.resolvePromise(true);
+			this.resolvePromise = undefined;
+		}
+		this.close();
+	}
+
+	private handleDeny() {
+		if (this.resolvePromise) {
+			this.resolvePromise(false);
+			this.resolvePromise = undefined;
+		}
+		this.close();
+	}
+
+	override close() {
+		super.close();
+		if (this.resolvePromise) {
+			this.resolvePromise(false);
+		}
+	}
+
+	protected override renderContent() {
+		return html`
+			${DialogContent({
+				children: html`
+					${DialogHeader({
+						title: i18n("Storage Permission Required"),
+						description: i18n("This app needs persistent storage to save your conversations"),
+					})}
+
+					<div class="mt-4 flex flex-col gap-4">
+						<div class="flex gap-3 p-4 bg-warning/10 border border-warning/20 rounded-lg">
+							<div class="flex-shrink-0 text-warning">
+								<svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+									<path d="M10.29 3.86L1.82 18a2 2 0 0 0 1.71 3h16.94a2 2 0 0 0 1.71-3L13.71 3.86a2 2 0 0 0-3.42 0z"></path>
+									<line x1="12" y1="9" x2="12" y2="13"></line>
+									<line x1="12" y1="17" x2="12.01" y2="17"></line>
+								</svg>
+							</div>
+							<div class="text-sm">
+								<p class="font-medium text-foreground mb-1">${i18n("Why is this needed?")}</p>
+								<p class="text-muted-foreground">
+									${i18n(
+										"Without persistent storage, your browser may delete saved conversations when it needs disk space. Granting this permission ensures your chat history is preserved.",
+									)}
+								</p>
+							</div>
+						</div>
+
+						<div class="text-sm text-muted-foreground">
+							<p class="mb-2">${i18n("What this means:")}</p>
+							<ul class="list-disc list-inside space-y-1 ml-2">
+								<li>${i18n("Your conversations will be saved locally in your browser")}</li>
+								<li>${i18n("Data will not be deleted automatically to free up space")}</li>
+								<li>${i18n("You can still manually clear data at any time")}</li>
+								<li>${i18n("No data is sent to external servers")}</li>
+							</ul>
+						</div>
+					</div>
+
+					<div class="mt-6 flex gap-3 justify-end">
+						${Button({
+							variant: "outline",
+							onClick: () => this.handleDeny(),
+							disabled: this.requesting,
+							children: i18n("Continue Anyway"),
+						})}
+						${Button({
+							variant: "default",
+							onClick: () => this.handleGrant(),
+							disabled: this.requesting,
+							children: this.requesting ? i18n("Requesting...") : i18n("Grant Permission"),
+						})}
+					</div>
+				`,
+			})}
+		`;
+	}
+}