perf: fix N+1 query pattern across all modules + rack numbering

CRITICAL PERF BUG: Every hook did storage.list() (1 HTTP call fetching ALL
items with values, discarding values, returning only keys) then storage.get()
for EACH key (N individual HTTP calls re-fetching values one by one).

With 6 entries + contacts + tags, Registratura page fired ~40 sequential
HTTP requests on load, where 3 would suffice.

Fix: Replace list()+N*get() with single exportAll() call in ALL hooks:
- registratura/registry-service.ts (added exportAll to RegistryStorage interface)
- address-book/use-contacts.ts
- it-inventory/use-inventory.ts
- password-vault/use-vault.ts
- word-templates/use-templates.ts
- prompt-generator/use-prompt-generator.ts
- hot-desk/use-reservations.ts
- email-signature/use-saved-signatures.ts
- digital-signatures/use-signatures.ts
- ai-chat/use-chat.ts
- core/tagging/tag-service.ts (uses storage.export())

Additional fixes:
- registratura/use-registry.ts: addEntry uses optimistic local state update
  instead of double-refresh; closeEntry batches saves with Promise.all +
  single refresh
- server-rack.tsx: reversed slot rendering so U1 is at bottom (standard
  rack numbering, per user's physical rack)

Performance impact: ~90% reduction in HTTP requests on page load for all modules

src/modules/email-signature/hooks/use-saved-signatures.ts

@@ -1,23 +1,22 @@
-'use client';
+"use client";
 
-import { useState, useEffect, useCallback } from 'react';
-import { useStorage } from '@/core/storage';
-import { v4 as uuid } from 'uuid';
-import type { SignatureConfig, SavedSignature } from '../types';
+import { useState, useEffect, useCallback } from "react";
+import { useStorage } from "@/core/storage";
+import { v4 as uuid } from "uuid";
+import type { SignatureConfig, SavedSignature } from "../types";
 
 export function useSavedSignatures() {
-  const storage = useStorage('email-signature');
+  const storage = useStorage("email-signature");
   const [saved, setSaved] = useState<SavedSignature[]>([]);
   const [loading, setLoading] = useState(true);
 
   const refresh = useCallback(async () => {
     setLoading(true);
-    const keys = await storage.list();
+    const all = await storage.exportAll();
     const items: SavedSignature[] = [];
-    for (const key of keys) {
-      if (key.startsWith('sig:')) {
-        const item = await storage.get<SavedSignature>(key);
-        if (item) items.push(item);
+    for (const [key, value] of Object.entries(all)) {
+      if (key.startsWith("sig:") && value) {
+        items.push(value as SavedSignature);
       }
     }
     items.sort((a, b) => b.updatedAt.localeCompare(a.updatedAt));
@@ -26,26 +25,34 @@ export function useSavedSignatures() {
   }, [storage]);
 
   // eslint-disable-next-line react-hooks/set-state-in-effect
-  useEffect(() => { refresh(); }, [refresh]);
+  useEffect(() => {
+    refresh();
+  }, [refresh]);
 
-  const save = useCallback(async (label: string, config: SignatureConfig) => {
-    const now = new Date().toISOString();
-    const entry: SavedSignature = {
-      id: uuid(),
-      label,
-      config,
-      createdAt: now,
-      updatedAt: now,
-    };
-    await storage.set(`sig:${entry.id}`, entry);
-    await refresh();
-    return entry;
-  }, [storage, refresh]);
+  const save = useCallback(
+    async (label: string, config: SignatureConfig) => {
+      const now = new Date().toISOString();
+      const entry: SavedSignature = {
+        id: uuid(),
+        label,
+        config,
+        createdAt: now,
+        updatedAt: now,
+      };
+      await storage.set(`sig:${entry.id}`, entry);
+      await refresh();
+      return entry;
+    },
+    [storage, refresh],
+  );
 
-  const remove = useCallback(async (id: string) => {
-    await storage.delete(`sig:${id}`);
-    await refresh();
-  }, [storage, refresh]);
+  const remove = useCallback(
+    async (id: string) => {
+      await storage.delete(`sig:${id}`);
+      await refresh();
+    },
+    [storage, refresh],
+  );
 
   return { saved, loading, save, remove, refresh };
 }