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/lib/theme-css.ts

@@ -31,6 +31,7 @@ const CSS_VARIABLES: Record<keyof ThemePalette, string> = {
 	link_color: "link-color",
 	link_hover_color: "link-hover-color",
 	border_color: "border-color",
+	panel_border_color: "panel-border-color",
 	gradient_from: "gradient-from",
 	gradient_to: "gradient-to",
 };