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-hardcoded-text.mjs

@@ -0,0 +1,127 @@
+/**
+ * check-hardcoded-text.mjs
+ * Scans shared/layout components for hardcoded English UI strings that should
+ * be replaced with t() calls.
+ *
+ * Heuristic: a JSX text node is flagged when it:
+ *   - contains at least one space (multi-word)
+ *   - is longer than 3 characters
+ *   - starts with an uppercase letter or common English word
+ *   - is NOT already wrapped in {t(...)}
+ *   - is NOT a CSS class, URL, comment, code attribute, or aria-label value
+ *
+ * Usage: node scripts/check-hardcoded-text.mjs
+ * Exit code 1 if any potential hardcoded strings 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, '..');
+
+/** Directories to scan */
+const SCAN_DIRS = [
+  join(ROOT, 'src', 'components', 'shared'),
+  join(ROOT, 'src', 'components', 'layout'),
+];
+
+// Collect .tsx files
+function* walkFiles(dir) {
+  for (const entry of readdirSync(dir)) {
+    const full = join(dir, entry);
+    if (statSync(full).isDirectory()) {
+      yield* walkFiles(full);
+    } else if (/\.tsx$/.test(entry)) {
+      yield full;
+    }
+  }
+}
+
+/**
+ * Patterns that indicate the string is NOT a hardcoded UI label:
+ *  - Only digits/punctuation
+ *  - Looks like a URL, path, class name, CSS value
+ *  - Already inside {t(…)}
+ *  - Attribute values like className, href, src, id, type, etc.
+ */
+const SKIP_RE = [
+  /^[\d\s.,/:%-]+$/,          // pure numbers/punctuation
+  /^https?:\/\//,             // URLs
+  /^\/[a-z]/,                 // paths
+  /^[a-z][-a-z0-9]*$/,        // single lowercase word (CSS class, attr value)
+  /^[a-z][a-zA-Z0-9]*=[a-z]/, // key=value attrs
+];
+
+function shouldSkip(str) {
+  const trimmed = str.trim();
+  if (trimmed.length <= 3) return true;
+  if (!/\s/.test(trimmed)) return true;  // single word
+  if (!/[A-Z]/.test(trimmed[0])) return true; // doesn't start with uppercase
+  for (const re of SKIP_RE) if (re.test(trimmed)) return true;
+  return false;
+}
+
+/**
+ * Find JSX text content that is hardcoded English.
+ * Strategy: look for lines where text appears between JSX tags but is NOT
+ * wrapped in a {…} expression.
+ *
+ * Pattern: >  Some Text Here   <  (with optional leading whitespace)
+ */
+const JSX_TEXT_RE = />\s*([A-Z][^<{}\n]{3,}?)\s*</g;
+
+/**
+ * Also catch string literals used directly as prop values that look like
+ * display text: title="Some English Text" (but not className, id, etc.)
+ */
+const DISPLAY_PROP_RE = /(?:title|label|placeholder|aria-label|alt)="([^"]{4,})"/g;
+
+const findings = [];
+
+for (const dir of SCAN_DIRS) {
+  for (const file of walkFiles(dir)) {
+    const rel = relative(ROOT, file).replace(/\\/g, '/');
+    const content = readFileSync(file, 'utf8');
+    const lines = content.split('\n');
+
+    for (const [lineIdx, line] of lines.entries()) {
+      // Skip comment lines
+      if (/^\s*\/\//.test(line) || /^\s*\*/.test(line)) continue;
+      // Skip lines that are already pure t() calls
+      if (/\{t\(/.test(line)) continue;
+
+      // JSX text between tags
+      let m;
+      JSX_TEXT_RE.lastIndex = 0;
+      while ((m = JSX_TEXT_RE.exec(line)) !== null) {
+        const text = m[1].trim();
+        if (!shouldSkip(text)) {
+          findings.push({ file: rel, line: lineIdx + 1, text });
+        }
+      }
+
+      // Display props with raw English strings
+      DISPLAY_PROP_RE.lastIndex = 0;
+      while ((m = DISPLAY_PROP_RE.exec(line)) !== null) {
+        const text = m[1].trim();
+        if (!shouldSkip(text)) {
+          findings.push({ file: rel, line: lineIdx + 1, text: `[prop] ${text}` });
+        }
+      }
+    }
+  }
+}
+
+if (findings.length === 0) {
+  console.log('✓ No hardcoded UI text found in shared/layout components');
+  process.exit(0);
+} else {
+  console.warn(`⚠ Found ${findings.length} potential hardcoded string(s):\n`);
+  for (const { file, line, text } of findings) {
+    console.warn(`  ${file}:${line}  →  "${text}"`);
+  }
+  // Exit 1 to allow failing CI; change to process.exit(0) to make it advisory only
+  process.exit(1);
+}