feat: make panel border color configurable via theme editor

- Add panel_border_color to THEME_COLOR_KEYS and base palettes
- Register panel-border-color CSS variable
- Fetch and inject panel_border_color in ThemeVars
- Add Panel border field to admin theme page
- Replace hardcoded #e9b124 with var(--panel-border-color) in UserView

src/app/admin/theme/page.tsx

@@ -87,6 +87,12 @@ const COLOR_FIELDS: {
 		def: "#eeb425",
 		desc: "Borders, dividers, outlines",
 	},
+	{
+		key: "panel_border_color",
+		label: "Panel border",
+		def: "#e9b124",
+		desc: "Border color of home page panels and cards",
+	},
 	{
 		key: "color_success",
 		label: "Success",