Refactor authentication and add file sharing with UTF-8 support

[solingerz]

This commit improves code quality and adds file sharing functionality with proper UTF-8 handling.

1. Authentication Refactoring

Extract duplicate authentication logic into a centralized utility module for better maintainability.

New file: utils/auth.ts

• AuthConfig interface for consistent auth configuration
• requireAuth() function with public read mode support
• requireAuthSimple() for APIs requiring strict authentication
• PUBLIC_READ_METHODS constant (GET, HEAD, PROPFIND)
• Proper WWW-Authenticate header handling

Refactored files:

• functions/api/share.ts
• functions/webdav/[[path]].ts
• src/Main.tsx
• src/app/share.ts

Benefits:

• Eliminates code duplication (DRY principle)
• Consistent authentication behavior across all endpoints
• Easier to maintain and test
• Better separation of concerns

2. UTF-8 Charset Handling

Fix character encoding issues for text files served via WebDAV.

Changes in functions/webdav/get.ts:

• Added isTextFile() to detect text files by MIME type and extension
• Added addUtf8Charset() to append charset=utf-8 to Content-Type headers
• Added addHtmlCharset() to inject into HTML files
• Supports common text formats: .txt, .html, .css, .js, .json, .xml, .md, .log, .csv, .ts, .jsx, .tsx, .vue, .py, .java, .c, .cpp, .h, .hpp, .php, .rb, .go, .rs, .swift, .kt, .scala

Changes in src/TextPadDrawer.tsx:

• Updated Blob and File creation to explicitly specify charset=utf-8

Impact:

• Ensures proper UTF-8 encoding for all text files
• Prevents character encoding issues in browsers
• Improves compatibility with international characters

3. File Sharing Feature

Implement secure file sharing with token-based pre-authorization.

Features:

• Token-based authentication using Cloudflare KV storage
• Pre-authorized URLs that bypass normal authentication
• Secure random token generation with automatic TTL

New files:

• functions/api/share.ts: API endpoint for creating share tokens
• functions/s/[token].ts: Token validation and file serving handler
• src/app/share.ts: Frontend sharing logic and UI components

Integration:

• Added share button to file manager UI (src/Main.tsx)
• Copy-to-clipboard functionality for share links
• Proper error handling and user feedback

Technical details:

• Uses Cloudflare KV for token storage with TTL support
• Generates cryptographically secure random tokens
• Validates tokens and expiration before serving files
• Integrates with existing authentication system

Files Changed:

• functions/api/share.ts (new)
• functions/s/[token].ts (new)
• functions/webdav/get.ts (modified)
• functions/webdav/[[path]].ts (modified)
• src/Main.tsx (modified)
• src/TextPadDrawer.tsx (modified)
• src/app/share.ts (new)
• utils/auth.ts (new)