refactor: extract API handlers into internal packages

Move HTTP handler logic out of main into internal api and server packages,
slimming main to a thin entrypoint. Bundles bug fixes: login nil-panic,
empty-password validation, chirp sort ordering, duplicate-email 409, and
restricting the file server to the assets directory.

run.sh

@@ -1,2 +1,7 @@
-go build -o ./bin
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd "$(dirname "$0")"
+
+go build -o ./bin/chirpy .
 ./bin/chirpy