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>

package.json

@@ -7,8 +7,9 @@
     "scripts": {
         "dev": "vite dev",
         "build": "vite build",
-        "build:prod": "cp .env.prod .env.production && vite build",
-        "build:staging": "cp .env.staging .env.production && vite build",
+        "build:dev": "vite build --mode dev",
+        "build:test": "vite build --mode test",
+        "build:prod": "vite build --mode prod",
         "preview": "vite preview",
         "test": "npm run test:integration && npm run test:unit",
         "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
@@ -17,7 +18,8 @@
         "format": "prettier --write .",
         "test:integration": "playwright test",
         "test:unit": "vitest",
-        "deploy:staging": "docker compose -f ../aether_container_env/docker-compose.yml build ae_app && docker compose -f ../aether_container_env/docker-compose.yml up -d ae_app",
+        "deploy:dev": "docker compose -f ../aether_container_env/docker-compose.yml build ae_app && docker compose -f ../aether_container_env/docker-compose.yml up -d ae_app",
+        "deploy:test": "docker compose -f ../aether_container_env/docker-compose.yml build --build-arg BUILD_MODE=test ae_app && docker compose -f ../aether_container_env/docker-compose.yml up -d ae_app",
         "deploy:prod": "docker compose -f ../aether_container_env/docker-compose.yml build --build-arg BUILD_MODE=prod ae_app && docker compose -f ../aether_container_env/docker-compose.yml up -d --remove-orphans ae_app",
         "compose:down": "docker compose -f ../aether_container_env/docker-compose.yml --profile database down"
     },