fix: Use solid RGB colors for modal and menu backgrounds

Problem: CSS variables --rpg-bg and --rpg-accent have 0.9 opacity,
making all modals and menus transparent even with gradient backgrounds.

Solution: Use solid RGB colors directly in gradients:
- rgba(22, 33, 62, 1) instead of var(--rpg-accent)
- rgba(26, 26, 46, 1) instead of var(--rpg-bg)

Changes:
- style.css: .rpg-modal-content uses solid gradient
- tabContextMenu.js: Context menu uses solid gradient
- Both maintain inset shadow for depth
- Mobile long-press support included

Now modals and context menus have opaque backgrounds matching
widget styling instead of see-through transparency.

style.css

@@ -1479,13 +1479,13 @@ body:has(.rpg-panel.rpg-position-left) #sheld {
 
 /* Modal Content Container */
 .rpg-modal-content {
-    background: var(--rpg-bg);
+    background: linear-gradient(135deg, rgba(22, 33, 62, 1) 0%, rgba(26, 26, 46, 1) 100%);
     border: 2px solid var(--rpg-border);
     border-radius: 8px;
     max-width: 600px;
     max-height: 80vh;
     overflow: auto;
-    box-shadow: 0 4px 20px var(--rpg-shadow);
+    box-shadow: 0 4px 18px var(--rpg-shadow), inset 0 0 12px rgba(0, 0, 0, 0.3);
 }
 
 /* Modal Header */