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>

static/js/dashboard.js

@@ -36,32 +36,5 @@ async function loadDashboard() {
             </tr>`).join('');
     }
 
-    // Persist quota support flag for shares.js to read
-    window._quotaSupported = d.quota_supported !== false;
-    window._quotaReason = d.quota_reason || '';
-
-    const qb = document.getElementById('dashQuotasBody');
-    if (!d.quota_supported) {
-        qb.innerHTML = `<tr><td colspan="4" style="color:var(--muted); font-size:.87rem">${esc(d.quota_reason)}</td></tr>`;
-    } else if (!d.quotas?.length) {
-        qb.innerHTML = `<tr><td colspan="4" style="color:var(--muted)">No quotas set.</td></tr>`;
-    } else {
-        qb.innerHTML = d.quotas.map(q => {
-            const p = Math.min(q.percent, 100);
-            const col = p > 90 ? 'var(--red)' : p > 75 ? 'var(--yellow)' : 'var(--green)';
-            return `<tr>
-                <td>${esc(q.user)}</td>
-                <td>${q.used_gb} GB</td>
-                <td>${q.soft_gb} GB</td>
-                <td>
-                    <div class="progress" style="height:8px">
-                        <div class="progress-bar" style="width:${p}%; background:${col}; transition:width .6s ease"></div>
-                    </div>
-                    <div class="mt-1" style="font-size:.75rem; color:var(--muted)">${p.toFixed(1)}%</div>
-                </td>
-            </tr>`;
-        }).join('');
-    }
-
     _startRefreshCountdown();
 }