fix: wire dark-mode theme tokens for all three tenant themes (Z1)

theme-engine.service.ts already sets [data-theme-mode] on the root, but
no CSS anywhere consumed it - picking Dark/System never changed anything
visually. Adds a structural dark override block per theme (dexar/lavero/
novo): bg/text/border/shadow tokens only. Brand colors (primary/secondary/
accent/gradients) are left untouched - a distinct dark-mode brand palette
is a design decision for the theme owner, not made here.

Also verified during this pass, no change needed (GAPS-AND-IMPROVEMENTS.md
was stale on these):
- og:locale already reads languageService.currentLanguage() dynamically
- SeoService.setItemMeta() is already called from product-details-container
- stars.component.scss already uses var(--border-color), no literal hex
- sellerId is already typed UUID, no bare-string field remains

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/styles/themes/dexar.theme.scss

@@ -39,6 +39,27 @@
   --radius-full: 999px;
 }
 
+// Structural dark-mode overrides only (surface/text/border tokens). Brand
+// colors (primary/secondary/accent/gradients) are unchanged - a distinct
+// dark-mode brand palette is a design decision, not made here. See
+// GAPS-AND-IMPROVEMENTS.md item "Dark mode selector does nothing".
+:root[data-theme-mode="dark"] {
+  --text-primary: #eef4f3;
+  --text-secondary: #b7c4c2;
+  --text-light: #8a9c9a;
+
+  --bg-primary: #10201e;
+  --bg-secondary: #16302c;
+  --bg-tertiary: #1c3a35;
+  --bg-header: rgba(0, 0, 0, 0.35);
+
+  --border-color: #2c4a45;
+  --border-dark: #3f6660;
+  --shadow-sm: 0 2px 8px rgba(0, 0, 0, 0.4);
+  --shadow-md: 0 4px 12px rgba(0, 0, 0, 0.45);
+  --shadow-lg: 0 12px 32px rgba(0, 0, 0, 0.55);
+}
+
 body {
   font-family: "DM Sans", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
 }