Use STORYKIT_PORT for both backend and frontend, add .story_kit_port to ignore files

Single env var STORYKIT_PORT configures backend port, frontend proxy target,
frontend dev server port (port + 2172), and WebSocket host. Added .story_kit_port
to .gitignore and .ignore to prevent git tracking and cargo watch restart loops.

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

.ignore

@@ -4,3 +4,4 @@ node_modules/
 .claude/
 .story_kit/
 store.json
+.story_kit_port