feat: format empty partitions on system disk (v0.32.6)

Detect and offer to format empty (no filesystem) partitions on the system
disk. Adds IsSystemPartition() for granular per-partition safety checks
instead of blocking the entire system disk. Init wizard shows formatable
partitions with appropriate warnings. Add felhotest demo node to docs.

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

CONTEXT.md

@@ -40,7 +40,7 @@ Last updated: 2026-02-19 (session 59)
 - **v0.11.8:** ✅ COMPLETE — Per-App Cross-Drive Backup (3-2-1 rule): rsync/restic to secondary drive, deploy page UI, backup page summary, scheduler jobs, API endpoints
 - **v0.11.9:** ✅ COMPLETE — UI Polish Fixes: spacing, tooltip on "Módszer", status dot instead of disabled checkbox, progressive disclosure, emoji cleanup
 - **First app deployed:** Paperless-ngx on demo-felhom.eu (2026-02-13)
-- **Running on:** demo-felhom (N100 mini PC) at 192.168.0.162:8080
+- **Running on:** demo-felhom (N100 mini PC) at 192.168.0.162:8080, felhotest (remote VPS) at router.abonet.hu:33022
 - **All Phase 1-5 features working:** deploy, start/stop/restart/update, logs, health-aware states, auth, monitoring, backups, backup detail page, system monitoring page, settings page
 
 ## Architecture decisions