feat(discord): add configurable presence style presets

Replace the hardcoded "Mining and crafting (Anki cards)" meme message
with a preset system. New `discordPresence.presenceStyle` option
supports four presets: "default" (clean bilingual), "meme" (the OG
Minecraft joke), "japanese" (fully JP), and "minimal". The default
preset shows "Sentence Mining" with 日本語学習中 as the small image
tooltip. Existing users can set presenceStyle to "meme" to keep the
old behavior.

src/types/config.ts

@@ -273,6 +273,7 @@ export interface ResolvedConfig {
   };
   discordPresence: {
     enabled: boolean;
+    presenceStyle: import('./integrations').DiscordPresenceStylePreset;
     updateIntervalMs: number;
     debounceMs: number;
   };