fix(pos): tema consistente en todas las paginas + theme bar ocultada

- app-init.js: aplica tema guardado en localStorage, oculta theme-bars
  que tapaban contenido (position:fixed), sobrescribe setTheme() de
  cada pagina para usar version persistente
- sidebar.js: toggle de tema (sol/luna) integrado en el sidebar
- Tema se mantiene al navegar entre paginas

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/static/js/app-init.js

@@ -127,6 +127,31 @@
         });
     });
 
+    // ─── Theme management ───
+    // Persist theme in localStorage, apply on load
+    var savedTheme = localStorage.getItem('pos_theme') || 'industrial';
+    document.documentElement.setAttribute('data-theme', savedTheme);
+
+    // Hide all theme bars (they overlap content with position:fixed)
+    document.querySelectorAll('.theme-bar').forEach(function(bar) {
+        bar.style.display = 'none';
+    });
+
+    // Expose theme toggle function (global)
+    window.posSetTheme = function(theme) {
+        document.documentElement.setAttribute('data-theme', theme);
+        localStorage.setItem('pos_theme', theme);
+    };
+
+    // Override any page-level setTheme functions so they use our persistent version
+    window.setTheme = window.posSetTheme;
+
+    // Also prevent any DOMContentLoaded theme switchers from overriding
+    // by re-applying our saved theme after a tick
+    setTimeout(function() {
+        document.documentElement.setAttribute('data-theme', savedTheme);
+    }, 100);
+
     // ─── Expose globally ───
     window.POS_USER = {
         name: name,