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>

config/layouts/home.json

@@ -7,7 +7,8 @@
     ],
     "main": [
       { "component": "overview-summary" },
-      { "component": "task-progress-grid", "options": { "historyDays": 365 } }
+      { "component": "task-progress-grid", "options": { "historyDays": 365 } },
+      { "component": "task-gantt" }
     ]
   }
 }