refactor(build): replace staging/cp env hack with vite --mode per-environment

- Rename .env.staging → .env.dev (and .default template)
- Add .env.test.default for the test tier (test-api.oneskyit.com)
- build:staging → build:dev/test/prod using vite --mode <name>
- deploy:staging → deploy:dev; add deploy:test
- Dockerfile: ARG BUILD_MODE=dev; explicit .env.runtime copy per mode
- .dockerignore: rewritten (deduped); allow .env.dev/.env.test/.env.prod
- .gitignore: track .env.dev.default and .env.test.default
- Remove dead PUBLIC_AE_* imports from ae_stores.ts (ACCOUNT_ID, EVENT_ID,
  NO_ACCOUNT_ID_TOKEN, SPONSORSHIP_CFG_ID); sponsorship_cfg_id defaults to null
- Strip dead vars from .env.prod.default template (AE_CFG_ID, AE_APP_NODE_PORT,
  ACCOUNT_ID, EVENT_ID, SPONSORSHIP_CFG_ID, NO_ACCOUNT_ID_TOKEN)
- GUIDE__Development.md: build:staging → build:dev

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

documentation/GUIDE__Development.md

@@ -10,7 +10,7 @@
    - **Zero Tolerance:** If a task introduces even a single svelte-check warning or error, it must not be merged. Resolve all warnings before committing.
 2. **Type Safety:** Ensure interfaces in `src/lib/types/ae_types.ts` match backend schemas.
 3. **Reactivity Check:** Verify Svelte 5 runes (`$state`, `$derived`) are not creating race conditions with Dexie `liveQuery`.
-4. **Build Check:** For major changes, run `npm run build:staging` to ensure no SSR or build-time failures.
+4. **Build Check:** For major changes, run `npm run build:dev` to ensure no SSR or build-time failures.
 5. **Integration Tests:** For changes to badge print, event layouts, or auth/store logic, run the relevant Playwright test file(s):
    ```bash
    npx playwright test tests/event_badge_render.test.ts tests/event_badge_attendee_workflow.test.ts