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>

.dockerignore

@@ -1,45 +1,47 @@
-.svelte-kit
-.vite
-node_modules
-dist
-build
-.cache
-.DS_Store
-.vscode
-.idea
-.git
+# Build artifacts and local state
+.svelte-kit/
+.vite/
+node_modules/
+build/
+dist/
+.cache/
+
+# VCS and IDE
+.git/
 .gitignore
-coverage
-tests
-documentation
-backups
+.vscode/
+.idea/
+
+# OS junk
+.DS_Store
+.directory
+
+# Logs and temp files
 *.log
 *.bak
 *.tgz
-.env
-.env.*
-npm-debug.log
-package-lock.json.bak
-yarn-error.log
-/.cache
-/.parcel-cachenode_modules/
-build/
-.svelte-kit/
-.git/
-.env
-.env.*
-!.env.staging
-!.env.prod
-npm_deploy/
-test-results/
-test_results/
-documentation/
-backups/
-*.log
-*.bak
-.claude/
-.vscode/
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
 .pnpm-debug.log*
+
+# Test output and dev-only dirs
+tests/
+test-results/
+test_results/
+coverage/
+documentation/
+backups/
+.claude/
+
+# Deployment artifacts
+npm_deploy/
+package-lock.json.bak
+
+# Env files: exclude all live secrets, allow only the per-environment files needed for Docker builds.
+# .env.local is workstation-only and must never enter a container image.
+.env
+.env.*
+!.env.dev
+!.env.test
+!.env.prod