v0.56.0: Phase 4 — FileBrowser scoping + deploy DB-on-SSD note + monitoring descriptions

4A: scope FileBrowser bind to <drive>/appdata (recovery units + Tier 2 copies under
backups/ are no longer mounted into FileBrowser — customer can't browse/delete the
thing that restores them). 4B: deploy storage-selection step states the chosen drive
holds files while the DB runs on the fast internal SSD + is backed up with the app.
4C: buildStorageBars stable sort + purpose description on the monitoring storage list.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/web/templates/monitoring.html

@@ -106,6 +106,7 @@
             <div class="system-bar">
                 <div class="system-bar-fill {{usageColor .Percent | printf "system-bar-%s"}}" style="width:{{printf "%.1f" .Percent}}%"></div>
             </div>
+            {{if .Purpose}}<div class="storage-purpose" style="font-size:.72rem;opacity:.65;margin-top:.2rem">{{.Purpose}}</div>{{end}}
         </div>
         {{end}}
         {{end}}