fix: header 3-col layout, shopping_name migration, demo mode UI, kiosk buttons left

Header layout:
- Redesign header-content as 3-column flex (left / center / right)
- Add #header-left div: dedicated slot for kiosk buttons (empty by default)
- header-title: flex-shrink auto, no more position:absolute centering hack
- header-actions: flex:1 1 0 + justify-content:flex-end (right)
- header-left: flex:1 1 0 (left) — equal width balances the title visually

Kiosk exit/refresh buttons:
- _injectKioskOverlay() now appends to #header-left instead of
  insertBefore(firstChild) — buttons appear on LEFT, not mixed with center

DB migration:
- Add shopping_name TEXT DEFAULT '' to CREATE TABLE products schema
- Add ALTER TABLE migration in migrateDB() for existing databases
- Avoids repeated ALTER TABLE in seed code on every request

Demo mode UI:
- _applyDemoModeUI(): hides ⚙️ settings nav button in demo mode
- Suppresses first-run setup wizard when _demoMode === true
- Shows a small DEMO badge in header-left
- Called from both syncSettingsFromDB() and _initApp()

api/database.php

@@ -39,6 +39,7 @@ function initializeDB(PDO $db): void {
             unit TEXT DEFAULT 'pz',
             default_quantity REAL DEFAULT 1,
             notes TEXT DEFAULT '',
+            shopping_name TEXT DEFAULT '',
             created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
             updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
         );
@@ -80,6 +81,9 @@ function migrateDB(PDO $db): void {
     if (!in_array('package_unit', $colNames)) {
         $db->exec("ALTER TABLE products ADD COLUMN package_unit TEXT DEFAULT ''");
     }
+    if (!in_array('shopping_name', $colNames)) {
+        $db->exec("ALTER TABLE products ADD COLUMN shopping_name TEXT DEFAULT ''");
+    }
 
     // Migrate transactions CHECK constraint to allow 'waste' type
     $sql = $db->query("SELECT sql FROM sqlite_master WHERE type='table' AND name='transactions'")->fetchColumn();