feat: add vocabulary browser with filters, audio playback, and example sentences

- Add Vocabularies page with searchable, level-grouped grid
- Implement multi-faceted filter system (word type, verb class, transitivity, reading ending, character count)
- Add male/female voice audio playback via WaniKani pronunciation data
- Display context sentences from WaniKani API
- Collapsible mnemonics section in detail modal
- Component kanji chips navigate to Collection with pre-filled search
- Vocabulary sync uses upsert to keep data fresh (audio, sentences, normalized POS)
- Add Vocabulary model, controller, service, and API route
- Add seed data with 28 vocabulary entries including transitive/intransitive verb pairs
- Full i18n support (EN, DE, JA) for all new UI elements

server/src/models/Vocabulary.js

@@ -0,0 +1,31 @@
+import mongoose from 'mongoose';
+
+const vocabularySchema = new mongoose.Schema({
+	userId: { type: mongoose.Schema.Types.ObjectId, ref: 'User', required: true },
+	wkSubjectId: { type: Number, required: true },
+	characters: { type: String, required: true },
+	meanings: { type: [String], default: [] },
+	readings: [{
+		reading: String,
+		primary: { type: Boolean, default: false }
+	}],
+	partsOfSpeech: { type: [String], default: [] },
+	level: { type: Number, required: true },
+	meaningMnemonic: { type: String, default: '' },
+	readingMnemonic: { type: String, default: '' },
+	componentSubjectIds: { type: [Number], default: [] },
+	pronunciationAudios: [{
+		url: String,
+		contentType: String,
+		gender: String
+	}],
+	contextSentences: [{
+		ja: String,
+		en: String
+	}]
+});
+
+vocabularySchema.index({ userId: 1, wkSubjectId: 1 }, { unique: true });
+vocabularySchema.index({ userId: 1, level: 1 });
+
+export const Vocabulary = mongoose.model('Vocabulary', vocabularySchema);