feat(i18n): update translations and improve error handling messages

- Updated site tagline and footer description in multiple languages.
- Enhanced error messages for various scenarios in the API service.
- Added translations for new error codes related to AI features and PDF processing.
- Improved user feedback in the UI components by utilizing i18n for dynamic text.
- Refactored error handling in the API service to map backend error codes to user-friendly messages.

frontend/scripts/check-i18n-keys.mjs

@@ -0,0 +1,83 @@
+/**
+ * check-i18n-keys.mjs
+ * Scans all .ts/.tsx files in src/ and verifies every static t('key') call
+ * exists as a dot-path entry in src/i18n/en.json.
+ *
+ * Usage: node scripts/check-i18n-keys.mjs
+ * Exit code 1 if any missing keys are found.
+ */
+
+import { readFileSync, readdirSync, statSync } from 'node:fs';
+import { join, relative } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = fileURLToPath(new URL('.', import.meta.url));
+const ROOT = join(__dirname, '..');
+const SRC = join(ROOT, 'src');
+const EN_JSON = join(ROOT, 'src', 'i18n', 'en.json');
+
+// Load en.json and build a flat Set of all dot-paths
+function flattenKeys(obj, prefix = '') {
+  const keys = new Set();
+  for (const [k, v] of Object.entries(obj)) {
+    const path = prefix ? `${prefix}.${k}` : k;
+    if (v !== null && typeof v === 'object' && !Array.isArray(v)) {
+      for (const nested of flattenKeys(v, path)) keys.add(nested);
+    } else {
+      keys.add(path);
+    }
+  }
+  return keys;
+}
+
+const enJson = JSON.parse(readFileSync(EN_JSON, 'utf8'));
+const definedKeys = flattenKeys(enJson);
+
+// Collect all .ts/.tsx files under src/
+function* walkFiles(dir) {
+  for (const entry of readdirSync(dir)) {
+    const full = join(dir, entry);
+    if (statSync(full).isDirectory()) {
+      yield* walkFiles(full);
+    } else if (/\.(tsx?|jsx?)$/.test(entry) && !entry.endsWith('.d.ts')) {
+      yield full;
+    }
+  }
+}
+
+// Extract static string arguments from t('...') or t("...") calls.
+// Matches: t('key'), t("key"), t(`key`), useTranslation().t('key'),
+// as well as i18n.t('key') patterns.
+const T_CALL_RE = /\bt\(\s*(['"`])([^'"`\s]+)\1/g;
+
+const missing = [];
+
+for (const file of walkFiles(SRC)) {
+  const rel = relative(ROOT, file).replace(/\\/g, '/');
+  const content = readFileSync(file, 'utf8');
+  const lines = content.split('\n');
+
+  for (const [lineIdx, line] of lines.entries()) {
+    let match;
+    T_CALL_RE.lastIndex = 0;
+    while ((match = T_CALL_RE.exec(line)) !== null) {
+      const key = match[2];
+      // Skip dynamic keys (contain ${) or non-string patterns
+      if (key.includes('${') || key.includes('(')) continue;
+      if (!definedKeys.has(key)) {
+        missing.push({ file: rel, line: lineIdx + 1, key });
+      }
+    }
+  }
+}
+
+if (missing.length === 0) {
+  console.log('✓ All t() keys are present in en.json');
+  process.exit(0);
+} else {
+  console.error(`✗ Found ${missing.length} missing i18n key(s):\n`);
+  for (const { file, line, key } of missing) {
+    console.error(`  ${file}:${line}  →  "${key}"`);
+  }
+  process.exit(1);
+}