Implement complete i18n translation for all UI text

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This PR implements comprehensive internationalization (i18n) support for the Rail Sign Generator application by replacing all hardcoded text with translation keys and providing translations for 5 languages.

Changes Made

Translation Files Updated

• Enhanced en.json, zh-Hans.json, zh-Hant.json, ja.json, and ko.json with complete translation coverage
• Added organized translation keys with proper namespacing (e.g., blocks.styles.specials.*)
• Added new translation categories for prefixes, line types, and UI elements

Component Updates

• window-header.tsx: Replaced hardcoded "Rail Sign Generator" title with t('head_bar.project_name')
• signs.tsx: Complete i18n implementation including:
  • Imported useTranslation hook
  • Replaced all hardcoded Chinese text in specialStyleConfigs with translation keys
  • Updated UI buttons, labels, and section headers to use translation keys
  • Made prefix text for signage (To/开往, Next Station/下一站, Terminus/终点站) translatable
  • Updated footer copyright text with proper link handling

Languages Supported

• English (en) - Default language
• Simplified Chinese (zh-Hans) - 简体中文
• Traditional Chinese (zh-Hant) - 繁體中文
• Japanese (ja) - 日本語
• Korean (ko) - 한국어

Translation Coverage

All user-facing text is now translatable:

• Window header and main title
• UI buttons ("Add New Block", "Download PNG", "Background Color")
• Block configuration options ("Block", "Dividing Line", "Style")
• Style selections ("Single Grid", "Exit Logo", "Toilet", "Line", etc.)
• Special attribute labels ("Text Alignment", "Line Type", "Line Number", etc.)
• Alignment options ("Align Right", "Align Center", "Align Left")
• Line type options ("Normal Line", "Loop Line", "Terminal Station")
• Yes/No radio button options
• Sign prefix text that appears in generated graphics
• Footer copyright notice

Technical Implementation

• Uses useTranslation hook from react-i18next following existing patterns
• Translation objects are properly reactive to language changes
• Maintains full compatibility with existing rmg-runtime language switching system
• All builds, tests, and linting continue to pass
• No breaking changes to existing functionality

Language switching works through the RMG runtime system by setting rmg-runtime-language in localStorage or using URL parameters.

Fixes #6.

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