Fix L22/L24/L27: kind-aware form, shell openUrl, Enter-to-download

L22: DownloadOptionsForm accepts optional `kind` prop. When set, hides
     audio-specific fields (Audio format) for video downloads and vice versa
     (Video container, Preferred codec) for audio downloads. Settings passes
     defaultKind so the defaults card shows only relevant options.

L24: Replace lone window.open('htmlUrl', '_blank') with a new openUrl IPC
     channel (shell:open-url) that calls shell.openExternal after validating
     the protocol is http/https. Consistent with how all other external opens
     work in the app. Preload + main handler + mock updated.

L27: DownloadBar URL-field Enter is now smart -- if formats/playlist are
     already loaded (or probe errored), Enter triggers download; otherwise
     it probes. Eliminates the keyboard dead-end where Enter never downloads.

typecheck + 242 tests + eslint green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/preload/index.ts

@@ -77,6 +77,8 @@ const api = {
 
   openPath: (path: string): Promise<string> => ipcRenderer.invoke(IpcChannels.openPath, path),
 
+  openUrl: (url: string): Promise<void> => ipcRenderer.invoke(IpcChannels.openUrl, url),
+
   showInFolder: (path: string): Promise<void> => ipcRenderer.invoke(IpcChannels.showInFolder, path),
 
   readClipboard: (): Promise<string> => ipcRenderer.invoke(IpcChannels.clipboardRead),