feat(ai): server-side chat history + assistant UX fixes

Server-side, per-user chat history (the thread now survives reload):
- new ai_chat_messages table + migration; getChatHistory/appendChatMessages/
  clearChatHistory; GET/POST/DELETE /api/admin/ai/history (ai.use-guarded);
  service + HTTP tests (isolation, ordering, clear, perm, validation).
- DashAssistant loads the thread on mount and persists each turn; "Vymazat"
  clears it. gcTime:0 so each mount re-reads the DB (no stale-cache resurrection);
  composer gated until history settles + seed flag flipped on submit (no
  seed/submit race clobbering a freshly-sent turn).

UX + correctness fixes from review/feedback:
- review cards moved OUT of the scrollable thread into a "Faktury k potvrzení"
  section so Uložit is always reachable; chat auto-scroll no longer fires on
  field edits.
- chat bubble text colour set on the Typography (GlobalStyles pins `p` to
  text.secondary, which was overriding the inherited bubble colour → unreadable).
- never clear input/staged PDFs on a failed submit (rollback + keep), so a
  budget error can't wipe staged invoices; roll back the optimistic user turn.
- stable attachment ids (no crypto.randomUUID — undefined over plain HTTP).
- editable Datum splatnosti + Popis fields; client-side extraction summary.

Security: DashProfile clipboard copy now uses an execCommand fallback (works
over plain HTTP) and only shows the success toast when the copy actually
succeeded — previously it falsely claimed to copy 2FA codes / the TOTP secret.

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

prisma/migrations/20260608130000_add_ai_chat_history/migration.sql

@@ -0,0 +1,11 @@
+-- AI assistant (Phase 1): per-user chat history (server-side, permanent).
+
+CREATE TABLE `ai_chat_messages` (
+  `id` INTEGER NOT NULL AUTO_INCREMENT,
+  `user_id` INTEGER NOT NULL,
+  `role` VARCHAR(20) NOT NULL,
+  `content` TEXT NOT NULL,
+  `created_at` TIMESTAMP(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0),
+  PRIMARY KEY (`id`),
+  INDEX `idx_ai_chat_user` (`user_id`, `id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

prisma/schema.prisma

@@ -89,6 +89,16 @@ model ai_usage {
   @@index([created_at], map: "idx_ai_usage_created")
 }
 
+model ai_chat_messages {
+  id         Int      @id @default(autoincrement())
+  user_id    Int
+  role       String   @db.VarChar(20)
+  content    String   @db.Text
+  created_at DateTime @default(now()) @db.Timestamp(0)
+
+  @@index([user_id, id], map: "idx_ai_chat_user")
+}
+
 model bank_accounts {
   id             Int       @id @default(autoincrement())
   account_name   String?   @db.VarChar(255)