Add transport-agnostic RPC layer for proxy app

Introduces yaak-rpc (shared RPC infrastructure) and yaak-proxy-lib
(proxy app logic decoupled from any frontend). A single Tauri command
dispatches all RPC calls through a typed router, with TypeScript types
auto-generated via ts-rs and a generic rpc() function for type-safe
calls from the frontend.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/yaak-proxy/rpc.ts

@@ -0,0 +1,12 @@
+import { invoke } from "@tauri-apps/api/core";
+import type { RpcSchema } from "../../crates-proxy/yaak-proxy-lib/bindings/gen_rpc";
+
+type Req<K extends keyof RpcSchema> = RpcSchema[K][0];
+type Res<K extends keyof RpcSchema> = RpcSchema[K][1];
+
+export async function rpc<K extends keyof RpcSchema>(
+  cmd: K,
+  payload: Req<K>,
+): Promise<Res<K>> {
+  return invoke("rpc", { cmd, payload }) as Promise<Res<K>>;
+}