chore: add translation cleanup tooling and remove unused keys

- Add check-unused-translations.js script for detecting unused i18n keys
- Remove unused player keys: refresh_tracks, audio_tracks, playback_state, index
- Remove unused aspect_ratio section (7 keys)
- Update copilot-instructions.md with i18n guidelines (only edit en.json, Crowdin handles other languages)

scripts/check-unused-translations.js

@@ -0,0 +1,121 @@
+#!/usr/bin/env node
+/**
+ * Check for unused translation keys in en.json
+ * Usage: bun run scripts/check-unused-translations.js [--remove]
+ */
+
+const fs = require("node:fs");
+const path = require("node:path");
+const { execSync } = require("node:child_process");
+
+const TRANSLATION_FILE = path.join(__dirname, "../translations/en.json");
+const REMOVE_UNUSED = process.argv.includes("--remove");
+
+// Read translation file
+const translations = JSON.parse(fs.readFileSync(TRANSLATION_FILE, "utf8"));
+
+// Flatten nested keys
+function flattenKeys(obj, prefix = "") {
+  let keys = [];
+  for (const [key, value] of Object.entries(obj)) {
+    const fullKey = prefix ? `${prefix}.${key}` : key;
+    if (typeof value === "object" && value !== null && !Array.isArray(value)) {
+      keys = keys.concat(flattenKeys(value, fullKey));
+    } else {
+      keys.push(fullKey);
+    }
+  }
+  return keys;
+}
+
+// Search for key usage in codebase
+function isKeyUsed(key) {
+  try {
+    // Escape special regex characters in the key
+    const escapedKey = key.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+
+    // Search in TypeScript/TSX files
+    const result = execSync(
+      `git grep -l "${escapedKey}" -- "*.ts" "*.tsx" 2>nul || echo ""`,
+      {
+        encoding: "utf8",
+        cwd: path.join(__dirname, ".."),
+        maxBuffer: 10 * 1024 * 1024,
+      },
+    ).trim();
+
+    return result.length > 0;
+  } catch (_error) {
+    // If grep fails, assume key is used to be safe
+    return true;
+  }
+}
+
+// Remove nested key from object
+function removeNestedKey(obj, keyPath) {
+  const keys = keyPath.split(".");
+  const lastKey = keys.pop();
+  let current = obj;
+
+  for (const key of keys) {
+    if (!current[key]) return false;
+    current = current[key];
+  }
+
+  if (current[lastKey] !== undefined) {
+    delete current[lastKey];
+
+    // Clean up empty parent objects
+    if (Object.keys(current).length === 0 && keys.length > 0) {
+      removeNestedKey(obj, keys.join("."));
+    }
+    return true;
+  }
+  return false;
+}
+
+console.log("🔍 Checking for unused translation keys...\n");
+
+const allKeys = flattenKeys(translations);
+const unusedKeys = [];
+
+for (const key of allKeys) {
+  if (!isKeyUsed(key)) {
+    unusedKeys.push(key);
+  }
+}
+
+if (unusedKeys.length === 0) {
+  console.log("✅ All translation keys are being used!");
+  process.exit(0);
+}
+
+console.log(`Found ${unusedKeys.length} unused translation keys:\n`);
+for (const key of unusedKeys) {
+  console.log(`  ❌ ${key}`);
+}
+
+if (REMOVE_UNUSED) {
+  console.log("\n🗑️  Removing unused keys...");
+
+  let removed = 0;
+  for (const key of unusedKeys) {
+    if (removeNestedKey(translations, key)) {
+      removed++;
+    }
+  }
+
+  // Write back to file
+  fs.writeFileSync(
+    TRANSLATION_FILE,
+    `${JSON.stringify(translations, null, 2)}\n`,
+    "utf8",
+  );
+
+  console.log(`✅ Removed ${removed} unused translation keys from en.json`);
+} else {
+  console.log("\n💡 Run with --remove flag to remove these keys from en.json");
+  console.log(
+    "   Example: bun run scripts/check-unused-translations.js --remove",
+  );
+}