feat: harden auth for public deploy + gantt, duration units, docker packaging

Security (fail-secure for production):
- Refuse to start without PPM_PASSWORD/PPM_PASSWORD_HASH unless
  PPM_ALLOW_NO_AUTH=1 (local dev only)
- Add login rate limiting (5 failures / 60s per client), enforced inside
  SessionManager.login so it cannot be bypassed at the endpoint layer
- compose defaults PPM_COOKIE_SECURE=true for HTTPS behind reverse proxy
- Add .dockerignore to keep runtime data/out of the image

Features:
- Task timeline gantt on home page (week/month zoom, horizontal scroll,
  infinite-arrow for open-ended dates, CSS Grid layout)
- Duration metrics gain a second/minute/hour unit for input & display
  (canonical storage stays integer seconds)
- Per-metric progress bars on home page (no more merged average)
- Backfill historical records via date picker in the today-record card
- Admin panel: stats overview, JSON import/restore, full task list mgmt

Fixes:
- Nullable task startDate; summarize handles missing start date
- Task dialog cancel/close no longer submits the form
- Import validates whole backup first, rejects path traversal, rebuilds
  archives, and replaces stale files

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -5,3 +5,7 @@ PPM_PASSWORD=change-me
 PPM_PORT=8000
 PPM_COOKIE_SECURE=false
 
+# 仅本地开发：允许免密码启动（未设置密码时默认 fail-secure 拒绝启动）
+# 生产环境请勿设置，务必配置 PPM_PASSWORD 或 PPM_PASSWORD_HASH
+PPM_ALLOW_NO_AUTH=false
+