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/tauri.ts

@@ -107,8 +107,15 @@ interface WorkingDirectoryPayload {
   conversation_id?: string;
 }
 
-export function cleanupConversationTracking(conversationId: string) {
+export async function cleanupConversationTracking(conversationId: string) {
   connectedConversations.delete(conversationId);
+
+  // Clean up any temp files associated with this conversation
+  try {
+    await invoke("cleanup_temp_files", { conversationId });
+  } catch (error) {
+    console.error("Failed to cleanup temp files for conversation:", error);
+  }
 }
 
 export async function initializeTauriListeners() {