feat: tech debt — legacy cleanup, Pinia bootstrap, TS util layer

Part A — Legacy cleanup + MARKTEXT_VERSION fix:
- Remove 19 deprecated webpack/loader devDependencies (muya sub-build deps kept)
- Delete dead .electron-vue/ files (build.js, dev-runner.js, marktextEnvironment.js, preinstall.js)
- electron.vite.config.ts: inject MARKTEXT_VERSION/VERSION_STRING/IS_STABLE/GIT_HASH
  into main.define (global.*) and renderer.define (process.versions.*) — fixes undefined at runtime

Part B — Pinia foundation (3 stateless listener modules):
- Install pinia@3; initialize createPinia() in renderer/main.js
- Create src/renderer/store/pinia/{autoUpdates,notification,tweet}.ts
- Remove autoUpdates/notification/tweet Vuex modules from store/index.js
- Replace dispatch('LISTEN_FOR_*') with useXxxStore().listen() in app.vue

Part C — TypeScript utility layer:
- Convert 10 src/renderer/util/*.js → .ts with explicit types
- Convert 4 src/main/utils/*.js → .ts with explicit types
- Add DOM lib to tsconfig.json; add vite-env.d.ts for *.css?inline + missing @types

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