Remove quota feature (incompatible with ZFS-backed LXC)

Linux setquota/repquota require quota support on the underlying
filesystem. The target deployment is a Proxmox LXC on ZFS storage where
this is not available inside the container.

Removed: /api/quota endpoint, _quota_supported, _quota_usage,
_find_quota_fs, quotas.js, quota modal, dashboard quota table,
Quota button from share rows.

PLANNING.md updated: notes quota as out of scope with reason.

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

PLANNING.md

@@ -70,14 +70,11 @@ MicroNAS/
 - [x] **Disk usage** — free / used / total for `/srv` with progress bar.
 - [x] **All shares in one view** — SMB + NFS, paths, access, size on disk.
 - [x] **Per-share data used** — `du -sb` per share path.
-- [x] **Quota usage** — per-user progress bars from `repquota`.
 - [x] Auto-refresh every 30 s + manual refresh button.
 
 ### Phase 2.5 — UX improvements ✅
 - [x] UI redesign — GitHub-dark palette, custom CSS vars, better contrast
       and typography.
-- [x] Inline quota management — "Quota" button per share row opens a modal
-      pre-filled with the share's valid users.
 - [x] Toast notifications — replaced all `alert()`.
 - [x] Frontend split into `templates/` + `static/css/` + `static/js/`.
 - [x] User management tab — list users (uid ≥ 1000), create, delete,
@@ -108,3 +105,7 @@ MicroNAS/
 - Database (config files are the source of truth).
 - Internet exposure / TLS termination (LAN + Tailscale only).
 - Clustering / multi-host management.
+- **Disk quotas** — removed. Linux `setquota`/`repquota` require quota
+  support on the filesystem. The target deployment uses a ZFS-backed
+  Proxmox LXC where this is unavailable inside the container. Per-dataset
+  limits can be set from the Proxmox host with `zfs set quota=`.