fix(machines): register node by Headscale username, not numeric id

Headscale's RegisterNodeRequest.user proto field is a string that the
RegisterNode handler resolves with GetUserByName (a strict WHERE name=?
SQL match) — there is no numeric-id fallback. The Owner select in the
Register Machine Key dialog was sending user.id, so registration failed
with ErrUserNotFound whenever the display name and the numeric id
disagreed (which is always for unlinked Headscale users).

Send user.name instead.

Closes #532

Amp-Thread-ID: https://ampcode.com/threads/T-019e7ae4-6862-760c-a3e7-239350eab71d
Co-authored-by: Amp <amp@ampcode.com>

app/routes/machines/dialogs/new.tsx

@@ -52,7 +52,9 @@ export default function NewMachine(data: NewMachineProps) {
             onValueChange={(v) => form.setValue("user", v)}
             placeholder="Select a user"
             items={data.users.map((user) => ({
-              value: user.id,
+              // Headscale's v1/node/register endpoint resolves the owner by
+              // username via GetUserByName, so we must pass user.name (not id).
+              value: user.name,
               label: getUserDisplayName(user),
             }))}
           />