Add multi-account support

- Schema: account_id FK on checks and layout_fields; UNIQUE per-account on check_no and field_name
- DB: runtime migration recreates both tables to add account_id (assigns existing rows to account 1)
- Routes: GET /api/accounts lists all; GET /api/account/:id replaces hardcoded id=1; POST /api/account/setup always creates a new account and returns accountId
- checks.js: all queries scoped by account_id; POST requires account_id in body
- pdf.js: resolves account from check's account_id instead of id=1; layout fields fetched per-account
- import-mdb.js: always INSERTs a new account (never deletes existing); all records tagged with new accountId
- Frontend: account switcher in header; activeAccountId persisted to localStorage; all API calls pass account_id; switching accounts reloads checks; wizard and import auto-switch to newly created account

public/css/style.css

@@ -41,6 +41,18 @@ header {
 .header-brand { font-size: 15px; font-weight: 600; }
 .header-info { font-size: 12px; color: rgba(255,255,255,0.7); }
 .header-info strong { color: #fff; }
+.header-left { display: flex; align-items: center; gap: 10px; }
+
+.account-switcher {
+  background: rgba(255,255,255,0.15);
+  border: 1px solid rgba(255,255,255,0.3);
+  color: #fff;
+  border-radius: 4px;
+  padding: 2px 6px;
+  font-size: 12px;
+  cursor: pointer;
+}
+.account-switcher option { background: var(--header-bg); color: #fff; }
 
 /* ── Toolbar ── */
 .toolbar {