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/stores/claude.ts

@@ -25,6 +25,7 @@ export const claudeStore = {
   isProcessing: conversationsStore.isProcessing,
   grantedTools: conversationsStore.grantedTools,
   pendingRetryMessage: conversationsStore.pendingRetryMessage,
+  attachments: conversationsStore.attachments,
 
   // New conversation-aware subscriptions
   conversations: conversationsStore.conversations,
@@ -67,6 +68,12 @@ export const claudeStore = {
   saveScrollPosition: conversationsStore.saveScrollPosition,
   getScrollPosition: conversationsStore.getScrollPosition,
 
+  // Attachment management
+  addAttachment: conversationsStore.addAttachment,
+  removeAttachment: conversationsStore.removeAttachment,
+  clearAttachments: conversationsStore.clearAttachments,
+  getAttachments: conversationsStore.getAttachments,
+
   getGrantedTools: (): string[] => {
     let tools: string[] = [];
     conversationsStore.grantedTools.subscribe((t) => (tools = Array.from(t)))();
@@ -86,6 +93,7 @@ export const claudeStore = {
     conversationsStore.setWorkingDirectory("");
     conversationsStore.setProcessing(false);
     conversationsStore.revokeAllTools();
+    conversationsStore.clearAttachments();
     // Also clear history restoration
     clearHistoryRestore();
   },