Add emoji confetti bursts and component playground.

Support app.relay.emoji_confetti on jumbo emoji clicks with canvas physics,
per-emoji particle profiles, and a fixed overlay that does not affect scroll.
Also add a Vite playground for live component previews and theme hover preview.

src/app/hooks/useTheme.ts

@@ -1,4 +1,5 @@
 import { lightTheme } from 'folds';
+import { atom, useAtomValue } from 'jotai';
 import { createContext, useContext, useEffect, useMemo, useState } from 'react';
 import { onDarkFontWeight, onLightFontWeight } from '../../config.css';
 import { butterTheme, catppuccinMochaTheme, darkTheme, discordTheme, discordDarkerTheme, mochaTheme, silverTheme, stationeryDarkTheme, stationeryTheme, twilightTheme } from '../../colors.css';
@@ -6,6 +7,9 @@ import { settingsAtom } from '../state/settings';
 import { useSetting } from '../state/hooks/settings';
 import { pluginRegistry } from '../features/settings/plugins/PluginAPI';
 
+/** Ephemeral theme id while hovering theme options in settings (not persisted). */
+export const themePreviewIdAtom = atom<string | undefined>(undefined);
+
 export enum ThemeKind {
   Light = 'light',
   Dark = 'dark',
@@ -190,6 +194,16 @@ export const useActiveTheme = (): Theme => {
   return selectedTheme;
 };
 
+/** Active theme, or hover-preview theme when browsing the theme menu. */
+export const useDisplayTheme = (): Theme => {
+  const activeTheme = useActiveTheme();
+  const themes = useThemes();
+  const previewId = useAtomValue(themePreviewIdAtom);
+
+  if (!previewId) return activeTheme;
+  return themes.find((theme) => theme.id === previewId) ?? activeTheme;
+};
+
 const ThemeContext = createContext<Theme | null>(null);
 export const ThemeContextProvider = ThemeContext.Provider;