feat: add image attachments to the chat composer

Support pasting (Ctrl/Cmd+V), drag-and-drop, and an Attach button to add
PNG/JPEG/GIF/WebP images to a message, with thumbnail previews and
multi-image support.

Attachments are delivered to the session using pi's native ImageContent
format and are run through pi's own resizeImage so they match pi's inline
image limits exactly. Image content now renders inline in the transcript.

A per-message delivery toggle also lets users save attachments into the
workspace `.pi-web/paste` folder and reference them so the agent reads
them with its own tools.

The accepted HTTP upload size is configurable via PI_WEB_MAX_UPLOAD_BYTES
or the maxUploadBytes config value (default 64 MB).

Closes #13

src/server/sessiond.ts

@@ -15,8 +15,9 @@ import { TerminalService } from "./terminals/terminalService.js";
 import { registerTerminalRoutes } from "./terminals/terminalRoutes.js";
 import { getPiWebRuntimeComponent } from "./piWebStatus.js";
 import { SESSIOND_RUNTIME_CAPABILITIES } from "../shared/capabilities.js";
+import { maxUploadBytes } from "../config.js";
 
-const app = Fastify({ logger: true });
+const app = Fastify({ logger: true, bodyLimit: maxUploadBytes() });
 await app.register(fastifyWebsocket);
 
 const eventHub = new SessionEventHub();