feat(04-04): slug resolution + multi-project client dashboard + slug edit

- validate-slug API route: resolves clients.slug for Edge middleware
- proxy.ts: slug-first resolution (D-06) — tries slug then falls back to token
- client-view.ts: complete rewrite — getClientWithProjectsByToken + getProjectView
  - No quote_items, no payment amounts in client API (CLAUDE.md security invariants)
- client/[token]/page.tsx: multi-project dashboard — 1 project = direct view,
  2+ projects = shadcn Tabs with project names (D-09/D-10)
- edit/page.tsx: slug field with link preview + unique constraint error handling
- actions.ts: updateClient now persists slug, redirects on success/slug_taken

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

src/app/admin/clients/[id]/edit/page.tsx

@@ -11,10 +11,13 @@ import Link from "next/link";
 
 export default async function EditClientPage({
   params,
+  searchParams,
 }: {
   params: Promise<{ id: string }>;
+  searchParams: Promise<{ error?: string }>;
 }) {
   const { id } = await params;
+  const { error } = await searchParams;
   const [client] = await db.select().from(clients).where(eq(clients.id, id)).limit(1);
   if (!client) notFound();
 
@@ -31,6 +34,12 @@ export default async function EditClientPage({
 
       <h1 className="text-xl font-bold text-[#1a1a1a] mb-6">Modifica cliente</h1>
 
+      {error === "slug_taken" && (
+        <div className="mb-4 rounded-lg bg-red-50 border border-red-200 px-4 py-3 text-sm text-red-700">
+          Slug già in uso da un altro cliente. Scegline uno diverso.
+        </div>
+      )}
+
       <form
         action={async (fd: FormData) => {
           "use server";
@@ -64,6 +73,27 @@ export default async function EditClientPage({
           />
         </div>
 
+        <div className="space-y-1.5">
+          <Label htmlFor="slug">Slug personalizzato (opzionale)</Label>
+          <p className="text-xs text-[#71717a]">
+            Solo lettere minuscole, numeri e trattini (es. mario-rossi). Min 3, max 50 caratteri.
+          </p>
+          <Input
+            id="slug"
+            name="slug"
+            type="text"
+            defaultValue={client.slug ?? ""}
+            pattern="[a-z0-9-]{3,50}"
+            placeholder="mario-rossi"
+          />
+          <p className="text-xs text-[#71717a]">
+            Link cliente:{" "}
+            <span className="font-mono text-[#1a1a1a]">
+              /client/{client.slug || client.token}
+            </span>
+          </p>
+        </div>
+
         <div className="flex gap-3 pt-2">
           <Button type="submit">Salva modifiche</Button>
           <Button asChild variant="ghost">