feat: add temp file management system for file uploads (#62)

Implements the foundation for file upload support by adding a temp file
management system that tracks files per conversation.

- Add temp_manager.rs module with TempFileManager struct
- Add Tauri commands: save_temp_file, register_temp_file, get_temp_files,
  cleanup_temp_files, cleanup_all_temp_files, cleanup_orphaned_temp_files
- Clean up orphaned files from previous sessions on app startup
- Clean up temp files when conversation is deleted
- Store temp files in /tmp/hikari-uploads/ with unique UUIDs

src/lib/stores/conversations.ts

@@ -272,7 +272,7 @@ function createConversationsStore() {
       return newConv.id;
     },
 
-    deleteConversation: (id: string) => {
+    deleteConversation: async (id: string) => {
       ensureInitialized();
       const convs = get(conversations);
       const activeId = get(activeConversationId);
@@ -282,8 +282,8 @@ function createConversationsStore() {
         return false;
       }
 
-      // Clean up tracking for this conversation
-      cleanupConversationTracking(id);
+      // Clean up tracking for this conversation (including temp files)
+      await cleanupConversationTracking(id);
 
       conversations.update((c) => {
         c.delete(id);

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() {