Declare build modes and the Enroll command for Dev Updater

modes: [release, debug], release first as the default; build-apk.sh
already takes the mode as its last argument, which is how Dev Updater
passes it. enroll: goes through server/enroll-link.sh rather than the
binary directly, because the command is handed the app's mode too and
because the server may have been built in either profile -- the wrapper
ignores the word and takes whichever ai-server exists, release first.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

server/enroll-link.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# Prints an enrollment link for one more phone -- `ai-server --enroll-link`,
+# see that flag's help. This is what Dev Updater's Enroll button runs, from
+# the checkout root, with the *app's* build mode as a trailing argument
+# (every command a component declares gets it). The mode does not bear on
+# the server, so it is ignored: the link comes from whichever ai-server has
+# been built here, release first, and the message below is what the card
+# shows when neither exists.
+set -eu
+cd "$(dirname "$0")"
+for bin in target/release/ai-server target/debug/ai-server; do
+    if [ -x "$bin" ]; then
+        exec "$bin" --enroll-link
+    fi
+done
+echo "ai-server is not built here yet -- update the server component first" >&2
+exit 1