[glm-grade=A] feat: add disk-safety warning to setup wizard + health retention settings

C-grade round-1 blockers fixed:
- [HIGH] retention default 14d → 30d to match engine (health-checker.js:34)
- [MEDIUM] phantom 'Settings → Disk Safety' path removed
- [MEDIUM] dangling 'stats polling interval' bullet (no such control in modal)
- [LOW] exaggerated 'hundreds of MB' → 'tens of MB'
- [LOW] button label mismatch (real button is '💾 Disk')

GLM round 2 verified all 5 fixes landed; no new regressions; HTML balanced.

Pre-existing follow-up parked: disk-settings.json saved values not reloaded by engine on container restart (out of scope for this commit).

status/index.html

@@ -694,9 +694,9 @@
             <span style="font-size: 1.5rem; line-height: 1.2;">⚠️</span>
             <div style="font-size: 0.92rem; line-height: 1.55; color: var(--text);">
               <strong style="color: var(--warn-fg, #f39c12);">Disk Usage Note:</strong>
-              DashCaddy stores health check history, container statistics, and event logs.
-              On a busy server, this data can accumulate over time.
-              Set appropriate retention limits in <strong>Settings → Health</strong> to prevent disk fill.
+              DashCaddy stores up to <strong>500 health check entries per service</strong> (plus container statistics and event logs).
+              At high check frequencies this may consume significant disk space — on a host with many services the history file can grow to tens of megabytes.
+              Adjust the <strong>health check interval</strong>, <strong>max entries per service</strong>, and <strong>health retention period</strong> in <strong>Disk Safety</strong> (the 💾 Disk button in the top bar) to control disk usage.
             </div>
           </div>
         </div>
@@ -704,12 +704,12 @@
         <div style="margin-top: 16px; padding: 14px 16px; background: var(--card-bg); border-radius: 8px; border: 1px solid var(--border);">
           <strong style="font-size: 0.9rem;">📋 Recommended after setup</strong>
           <ul style="margin: 8px 0 0; padding-left: 20px; font-size: 0.85rem; color: var(--muted); line-height: 1.6;">
-            <li>Open <strong>Health → Configure → Global Settings</strong></li>
-            <li>Set a <strong>health check polling interval</strong> (default: 60s)</li>
-            <li>Set a <strong>stats polling interval</strong> (default: 30s)</li>
-            <li>Set a <strong>data retention period</strong> (default: 30 days)</li>
-            <li>Cap <strong>max entries per service</strong> (default: 500)</li>
-            <li>Set a <strong>disk-usage warning threshold</strong> (default: 80%)</li>
+            <li>Open the <strong>💾 Disk</strong> button in the top bar (opens the Disk Safety modal)</li>
+            <li>Set a <strong>health check polling interval</strong> (default: 30s)</li>
+            <li>Set a <strong>health retention period</strong> (default: 30 days)</li>
+            <li>Cap <strong>max health entries per service</strong> (default: 500)</li>
+            <li>Cap <strong>max stats entries</strong> (default: 500)</li>
+            <li>Click <strong>Clean Up Now</strong> to purge old data immediately</li>
           </ul>
         </div>