feat: add shared cross-platform catbox media upload flow

Added `useFileUpload()` and `uploadToCatbox()` to centralize upload behavior and remove duplicated upload code from `post-form.tsx` and `reply-modal.tsx`. The flow now routes Android to `FileUploader`, Electron to hidden-input + catbox upload, and web to a translated fallback alert with consistent upload UI state.

src/lib/utils/__tests__/catbox-utils.test.ts

@@ -0,0 +1,51 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { uploadToCatbox } from '../catbox-utils';
+
+describe('uploadToCatbox', () => {
+  const originalFetch = global.fetch;
+  beforeEach(() => {
+    global.fetch = originalFetch;
+  });
+
+  it('POSTs file to catbox API and returns trimmed URL', async () => {
+    const mockFile = new File(['content'], 'test.png', { type: 'image/png' });
+    vi.stubGlobal(
+      'fetch',
+      vi.fn().mockResolvedValue({
+        ok: true,
+        text: () => Promise.resolve('  https://files.catbox.moe/abc123.png  '),
+      }),
+    );
+
+    const url = await uploadToCatbox(mockFile);
+    expect(url).toBe('https://files.catbox.moe/abc123.png');
+
+    const call = (global.fetch as ReturnType<typeof vi.fn>).mock.calls[0];
+    expect(call[0]).toBe('https://catbox.moe/user/api.php');
+    expect(call[1]?.method).toBe('POST');
+    const body = call[1]?.body as FormData;
+    expect(body.get('reqtype')).toBe('fileupload');
+    expect(body.get('fileToUpload')).toBe(mockFile);
+  });
+
+  it('throws Error on non-ok response', async () => {
+    const mockFile = new File(['x'], 'x.txt', { type: 'text/plain' });
+    vi.stubGlobal(
+      'fetch',
+      vi.fn().mockResolvedValue({
+        ok: false,
+        status: 500,
+        statusText: 'Internal Server Error',
+      }),
+    );
+
+    await expect(uploadToCatbox(mockFile)).rejects.toThrow('Upload failed: 500 Internal Server Error');
+  });
+
+  it('throws when fetch rejects with a network error', async () => {
+    const mockFile = new File(['x'], 'x.txt', { type: 'text/plain' });
+    vi.stubGlobal('fetch', vi.fn().mockRejectedValue(new Error('Network down')));
+
+    await expect(uploadToCatbox(mockFile)).rejects.toThrow('Network down');
+  });
+});

src/lib/utils/catbox-utils.ts

@@ -0,0 +1,25 @@
+const CATBOX_API = 'https://catbox.moe/user/api.php';
+
+/**
+ * Upload a file to catbox.moe.
+ * @param file - The file to upload
+ * @returns The URL of the uploaded file
+ * @throws Error when the response is not ok
+ */
+export async function uploadToCatbox(file: File): Promise<string> {
+  const formData = new FormData();
+  formData.append('reqtype', 'fileupload');
+  formData.append('fileToUpload', file);
+
+  const response = await fetch(CATBOX_API, {
+    method: 'POST',
+    body: formData,
+  });
+
+  if (!response.ok) {
+    throw new Error(`Upload failed: ${response.status} ${response.statusText}`);
+  }
+
+  const text = await response.text();
+  return text.trim();
+}