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()

assets/css/style.css

@@ -105,17 +105,23 @@ body {
 }
 
 .header-content {
-    position: relative;
     display: flex;
     align-items: center;
-    justify-content: flex-end;
     width: 100%;
+    gap: 0;
+}
+
+/* Left zone: kiosk-only buttons (exit, refresh) — empty on normal browser */
+.header-left {
+    flex: 1 1 0;
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    min-width: 0;
 }
 
 .header-title {
-    position: absolute;
-    left: 50%;
-    transform: translateX(-50%);
+    flex: 0 1 auto;
     font-size: 1.3rem;
     font-weight: 700;
     cursor: pointer;
@@ -123,9 +129,10 @@ body {
     align-items: baseline;
     gap: 6px;
     white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
     pointer-events: auto;
-    /* prevent title overlapping action buttons on small screens */
-    max-width: calc(100% - 200px);
+    text-align: center;
 }
 
 .header-version {
@@ -209,9 +216,12 @@ body {
 }
 
 .header-actions {
+    flex: 1 1 0;
     display: flex;
     gap: 10px;
     align-items: center;
+    justify-content: flex-end;
+    min-width: 0;
 }
 
 /* ── Smart Scale status indicator in header ─────────────────────────── */