Remove labels from theme and style pickers in sidebar

- Remove "Theme:" label from theme selector dropdown
- Remove "Style:" label from style selector dropdown
- Create cleaner, more minimal sidebar UI

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/components/sidebar.rs

@@ -219,7 +219,6 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                 </div>
 
                 <div class="theme-selector">
-                    <label>{"Theme:"}</label>
                     <select class="theme-selector-dropdown" onchange={on_theme_change}>
                         <option value="default" selected={matches!(props.current_theme, Theme::Default)}>{"Default"}</option>
                         <option value="ocean" selected={matches!(props.current_theme, Theme::Ocean)}>{"Ocean"}</option>
@@ -233,7 +232,6 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                 </div>
 
                 <div class="style-selector">
-                    <label>{"Style:"}</label>
                     <select class="style-selector-dropdown" onchange={on_style_change}>
                         <option value="default" selected={matches!(props.current_style, Style::Default)}>{"Default"}</option>
                         <option value="google" selected={matches!(props.current_style, Style::Google)}>{"Google Calendar"}</option>