feat: add file upload and preview support (images and documents)

- Support image (JPEG/PNG/GIF/WebP, max 5MB) and document uploads
- Images sent as multimodal content blocks to Claude SDK
- Documents saved to temp dir for Claude's Read tool
- Emerald capsule badges in input for attached files
- Drag-and-drop, paste, and file picker support via PromptInput
- Chat display: image thumbnails with lightbox, file cards
- Lightbox uses Radix Dialog with prev/next navigation
- File metadata embedded in message content as HTML comments
- Compatible with existing slash command badge system

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/index.ts

@@ -392,6 +392,30 @@ export const SETTING_KEYS = {
   MAX_THINKING_TOKENS: 'max_thinking_tokens',
 } as const;
 
+// ==========================================
+// File Attachment Types
+// ==========================================
+
+export interface FileAttachment {
+  id: string;
+  name: string;
+  type: string; // MIME type
+  size: number;
+  data: string; // base64 encoded content
+}
+
+// Check if a MIME type is an image
+export function isImageFile(type: string): boolean {
+  return type.startsWith('image/');
+}
+
+// Format bytes into human-readable size
+export function formatFileSize(bytes: number): string {
+  if (bytes < 1024) return bytes + ' B';
+  if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB';
+  return (bytes / (1024 * 1024)).toFixed(1) + ' MB';
+}
+
 // ==========================================
 // Claude Client Types
 // ==========================================
@@ -406,4 +430,5 @@ export interface ClaudeStreamOptions {
   mcpServers?: Record<string, MCPServerConfig>;
   abortController?: AbortController;
   permissionMode?: string;
+  files?: FileAttachment[];
 }