feat: add multilingual font support for translated PDFs

- Add NotoSansKR and NotoSansThai fonts for Korean and Thai language support
- Update download_fonts.sh to download all required fonts
- Add LANGUAGE_FONT_MAP for language-to-font mapping in pdf_generator_service.py
- Add get_font_for_language() method to select appropriate font based on target language
- Update _get_reflow_styles() to accept target_lang parameter
- Pass target_lang through generate_translated_pdf() to PDF generation methods
- Fix garbled characters (亂碼) issue for Korean and Thai translations

Supported languages:
- Chinese (zh-CN/zh-TW), Japanese (ja): NotoSansSC
- Korean (ko): NotoSansKR
- Thai (th): NotoSansThai
- Russian, Vietnamese, Latin languages: NotoSansSC

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/download_fonts.sh

@@ -1,31 +1,56 @@
 #!/bin/bash
-# Download Noto Sans SC TrueType font for layout-preserving PDF generation
+# Download Noto Sans fonts for multilingual PDF generation
+# Supports: CJK (Chinese, Japanese, Korean), Thai, Vietnamese, Russian, etc.
 
 set -e
 
 FONT_DIR="backend/fonts"
-FONT_URL="https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansSC-VF.ttf"
-FONT_FILE="NotoSansSC-Regular.ttf"
 
-echo "🔤 Downloading Chinese font for PDF generation..."
+echo "🔤 Downloading fonts for multilingual PDF generation..."
 
 # Create font directory
 mkdir -p "$FONT_DIR"
 
-# Download font if not exists
-if [ -f "$FONT_DIR/$FONT_FILE" ]; then
-    echo "✓ Font already exists: $FONT_DIR/$FONT_FILE"
-else
-    echo "Downloading from GitHub..."
-    wget "$FONT_URL" -O "$FONT_DIR/$FONT_FILE"
+# Function to download font
+download_font() {
+    local url="$1"
+    local file="$2"
+    local desc="$3"
 
-    if [ -f "$FONT_DIR/$FONT_FILE" ]; then
-        SIZE=$(du -h "$FONT_DIR/$FONT_FILE" | cut -f1)
-        echo "✓ Font downloaded successfully: $SIZE"
+    if [ -f "$FONT_DIR/$file" ]; then
+        echo "✓ $desc already exists"
     else
-        echo "✗ Font download failed"
-        exit 1
+        echo "Downloading $desc..."
+        if wget -q "$url" -O "$FONT_DIR/$file"; then
+            SIZE=$(du -h "$FONT_DIR/$file" | cut -f1)
+            echo "✓ $desc downloaded: $SIZE"
+        else
+            echo "✗ Failed to download $desc"
+            return 1
+        fi
     fi
-fi
+}
 
+# NotoSansSC - Chinese (Simplified), also covers Japanese and basic CJK
+download_font \
+    "https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansSC-VF.ttf" \
+    "NotoSansSC-Regular.ttf" \
+    "Noto Sans SC (Chinese/Japanese)"
+
+# NotoSansKR - Korean
+download_font \
+    "https://github.com/notofonts/noto-cjk/raw/main/Sans/Variable/TTF/Subset/NotoSansKR-VF.ttf" \
+    "NotoSansKR-Regular.ttf" \
+    "Noto Sans KR (Korean)"
+
+# NotoSansThai - Thai
+download_font \
+    "https://github.com/notofonts/noto-fonts/raw/main/hinted/ttf/NotoSansThai/NotoSansThai-Regular.ttf" \
+    "NotoSansThai-Regular.ttf" \
+    "Noto Sans Thai"
+
+echo ""
 echo "✅ Font setup complete!"
+echo "Supported languages: Chinese (zh-CN/zh-TW), Japanese (ja), Korean (ko),"
+echo "                     Thai (th), Russian (ru), Vietnamese (vi),"
+echo "                     and all Latin-based languages (en, de, fr, es, etc.)"