chore: update deps and local cleanup

- update npm dependencies, including Vite 8.1.0 and TypeScript 6.0.3

- update bundled Deno engine locks and macOS arm64 binary to 2.8.3

- add npm run check:updates for npm and engine freshness checks

- ignore local agent/planning notes and remove tracked internal docs

.gitignore

@@ -8,6 +8,16 @@ skills-lock.json
 *.xcuserdata/
 *.xcuserstate
 
+# Local agent and planning notes
+AGENT.md
+AGENTS.md
+CLAUDE.md
+GEMINI.md
+implementation_plan.md
+CROSS_PLATFORM_CHECKLIST.md
+Cross-platform-checklist-gemini.MD
+YouTube_media_download_handoff.md
+
 # Frontend output and logs
 node_modules/
 dist/