refactor: Store pasted images in memory as base64 data URLs

- Remove save_clipboard_image and cleanup_temp_images backend commands
- Update FloatingPromptInput to store pasted images as data URLs in the prompt
- Update ImagePreview component to handle both file paths and data URLs
- Update extractImagePaths to properly handle quoted data URLs
- Update handleRemoveImage to handle data URL removal

This eliminates file system operations for pasted images and stores them directly
in the prompt as base64 data URLs (e.g., @"data:image/png;base64,...").
Images are now fully self-contained within the session without creating temp files.

src/components/ClaudeCodeSession.tsx

@@ -20,7 +20,6 @@ import { api, type Session } from "@/lib/api";
 import { cn } from "@/lib/utils";
 import { open } from "@tauri-apps/plugin-dialog";
 import { listen, type UnlistenFn } from "@tauri-apps/api/event";
-import { invoke } from "@tauri-apps/api/core";
 import { StreamMessage } from "./StreamMessage";
 import { FloatingPromptInput, type FloatingPromptInputRef } from "./FloatingPromptInput";
 import { ErrorBoundary } from "./ErrorBoundary";
@@ -828,16 +827,6 @@ export const ClaudeCodeSession: React.FC<ClaudeCodeSessionProps> = ({
         api.clearCheckpointManager(effectiveSession.id).catch(err => {
           console.error("Failed to clear checkpoint manager:", err);
         });
-        
-        // Clean up temporary images
-        if (projectPath) {
-          invoke('cleanup_temp_images', {
-            projectPath,
-            sessionId: effectiveSession.id
-          }).catch((err: any) => {
-            console.error("Failed to cleanup temp images:", err);
-          });
-        }
       }
     };
   }, [effectiveSession, projectPath]);