feat: pagina Conversazioni — inbox unificata messaggi clienti

Nuova pagina admin /admin/conversazioni: vista WhatsApp-style (lista
conversazioni a sinistra, thread + risposta a destra) che aggrega i
messaggi di tutti i clienti dalla tabella comments, cross-cliente.

- Migration additiva 0014: clients.admin_last_read_at per tracciare
  letto/non-letto (pallini + badge in sidebar). Applicata a prod.
- conversations-queries.ts: aggregazione entity_id → cliente
  (general/phase/task/deliverable), getConversations /
  getConversationThread / getUnreadConversationsCount.
- Risposte admin salvate come commento "general" (visibili anche nella
  chat cliente e nel CommentsTab della scheda).
- Voce di menu + badge non-letti (AdminSidebar/AdminShell/layout).
- Token semantici per dual light/dark; refresh manuale (no polling).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/db/schema.ts

@@ -37,6 +37,9 @@ export const clients = pgTable("clients", {
     "0"
   ),
   archived: boolean("archived").notNull().default(false),
+  // Conversazioni inbox: timestamp of the admin's last read of this client's
+  // conversation. NULL = never read (treated as unread). Set via markConversationRead.
+  admin_last_read_at: timestamp("admin_last_read_at", { withTimezone: true }),
   created_at: timestamp("created_at", { withTimezone: true })
     .notNull()
     .defaultNow(),