P3: transfer engine with queue, progress, cancel + transfer bar UI

- TransferEngine: FIFO queue, provider-generic stream pipeline (covers all four
  panel combos), throttled transferUpdate push, cancel via stream destroy,
  self-copy guards, failSession hook for dead SSH sessions
- TransfersService + TransferBar: per-job rows, progress bars, cancel/dismiss,
  auto-dismiss completed, dst panel refresh
- F5 + Copy button wired to active-panel selection
- verified: 30MB nested tree copy (UTF-8 names, empty dirs), self-copy rejection,
  mid-transfer cancel leaving partial file

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bun/rpc-handlers.ts

@@ -2,6 +2,7 @@ import { BrowserView } from "electrobun/bun";
 import type { AppRPC } from "../shared/rpc";
 import type { Result } from "../shared/types";
 import type { SessionRegistry } from "./fs/registry";
+import type { TransferEngine } from "./transfers/engine";
 import { removeRecursive } from "./transfers/walk";
 
 // SFTP ops can be slow (big listings, recursive deletes) — the 1s electrobun
@@ -18,7 +19,7 @@ async function wrap<T extends object>(fn: () => Promise<T> | T): Promise<Result<
 
 const NOT_YET = { ok: false as const, error: "Not implemented yet" };
 
-export function createAppRPC(registry: SessionRegistry) {
+export function createAppRPC(registry: SessionRegistry, engine: TransferEngine) {
 	return BrowserView.defineRPC<AppRPC>({
 		maxRequestTime: MAX_REQUEST_TIME_MS,
 		handlers: {
@@ -70,10 +71,21 @@ export function createAppRPC(registry: SessionRegistry) {
 				// P4: SFTP sessions
 				connect: async () => NOT_YET,
 				disconnect: async () => NOT_YET,
-				// P3: transfer engine
-				enqueueTransfers: async () => NOT_YET,
-				cancelTransfer: async () => NOT_YET,
-				dismissTransfer: async () => NOT_YET,
+
+				enqueueTransfers: (params) =>
+					wrap(async () => ({ jobIds: engine.enqueue(params) })),
+
+				cancelTransfer: ({ jobId }) =>
+					wrap(async () => {
+						engine.cancel(jobId);
+						return {};
+					}),
+
+				dismissTransfer: ({ jobId }) =>
+					wrap(async () => {
+						engine.dismiss(jobId);
+						return {};
+					}),
 				// P4: connection store
 				getConnections: async () => NOT_YET,
 				upsertConnection: async () => NOT_YET,