feat(dictionary): add Hachidori backend support

- Add backend selection, setup gating, Anki integration, and external host support
- Add launcher flags, documentation, packaging, and focused tests
- Open on-demand overlay modals on the first attempt

src/config/definitions.ts

@@ -22,6 +22,7 @@ export type {
 } from './definitions/shared';
 
 const {
+  dictionaryBackend,
   subtitlePosition,
   keybindings,
   websocket,
@@ -57,6 +58,7 @@ const { stats } = STATS_DEFAULT_CONFIG;
 
 export const DEFAULT_CONFIG: ResolvedConfig = {
   subtitleGeneration: { ...DEFAULT_SUBTITLE_GENERATION_CONFIG },
+  dictionaryBackend,
   subtitlePosition,
   keybindings,
   websocket,