feat(wasm): expose taildrive WebDAV server and listDrivePeers via JS bridge

Add drive.go (build tag !ts_omit_drive): implements drive.FileSystemForRemote
with a JS-backed handler. Streams request bodies chunk-by-chunk via
readBodyChunk() and response bodies via write()/end() callbacks so no
full-body buffering occurs regardless of file size. The handler is nil-safe:
returns 404 until setDriveHandler() is called from JS.

Add drive_stub.go (build tag ts_omit_drive): no-op stubs for stripped builds.

Add peer.go: extract buildPeerAPIURL helper (previously inline in run()).

Modify wasm_js.go: call initDriveForRemote before NewLocalBackend (SubSystem
is set-once), expose setDriveHandler and listDrivePeers via wireDriveJS,
and refactor the inline peerAPI URL logic to use buildPeerAPIURL.

listDrivePeers mirrors native driveRemotesFromPeers: returns empty if
DriveAccessEnabled() is false, then filters peers by PeerCapabilityTaildriveSharer
using lb.PeerCaps(addr).HasCapability() (the live ACL-derived cap map).

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

cmd/tsconnect/wasm/drive_stub.go

@@ -0,0 +1,27 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ts_omit_drive
+
+package main
+
+import (
+	"syscall/js"
+
+	"tailscale.com/tsd"
+)
+
+type jsFileSystemForRemote struct{}
+
+// initDriveForRemote is a no-op when the drive feature is omitted.
+func initDriveForRemote(_ *tsd.System) *jsFileSystemForRemote { return nil }
+
+// wireDriveJS is a no-op when the drive feature is omitted.
+func wireDriveJS(_ *jsIPN, _ *jsFileSystemForRemote, _ map[string]any) {}
+
+// listDrivePeers returns an empty list when the drive feature is omitted.
+func (i *jsIPN) listDrivePeers() js.Value {
+	return makePromise(func() (any, error) {
+		return "[]", nil
+	})
+}