feat: add native clipboard support for screenshot paste (#67)

## Summary
- Adds Tauri clipboard-manager plugin to read images from native clipboard
- Falls back to native clipboard when WebView clipboard API returns empty (fixes screenshot paste)
- Allows sending messages with just attachments (no text required)
- Logs attached files to output with 📎 emoji

## Test plan
- [ ] Build and run the app natively on Windows
- [ ] Copy a screenshot (Win+Shift+S) and paste in the chat input
- [ ] Verify the screenshot appears as an attachment preview
- [ ] Send the attachment and verify Claude receives the file path
- [ ] Test sending a message with only an attachment (no text)
- [ ] Verify the 📎 log line shows the attached filename

**Note:** Paste will not work in WSLg dev environment due to clipboard isolation - needs native Windows build to test.

✨ This PR was created with help from Hikari~ 🌸

Co-authored-by: Hikari <hikari@nhcarrigan.com>
Reviewed-on: #67
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

src/lib/types/messages.ts

@@ -142,6 +142,16 @@ export interface UserQuestionEvent {
 
 export type ConnectionStatus = "disconnected" | "connecting" | "connected" | "error";
 
+export interface Attachment {
+  id: string;
+  filename: string;
+  path: string;
+  size: number;
+  type: "image" | "document" | "other";
+  mimeType?: string;
+  previewUrl?: string; // For images, a data URL or object URL for preview
+}
+
 export interface UpdateInfo {
   current_version: string;
   latest_version: string;