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/main/index.ts

@@ -325,6 +325,14 @@ function registerIpcHandlers(): void {
   })
 
   ipcMain.handle(IpcChannels.openPath, (_e, p: string) => safeOpenPath(p))
+  ipcMain.handle(IpcChannels.openUrl, (_e, url: string) => {
+    try {
+      const { protocol } = new URL(url)
+      if (protocol === 'http:' || protocol === 'https:') void shell.openExternal(url)
+    } catch {
+      // Ignore malformed URLs.
+    }
+  })
   ipcMain.handle(IpcChannels.showInFolder, (_e, p: string) => safeShowInFolder(p))
 
   ipcMain.handle(IpcChannels.clipboardRead, () => clipboard.readText())