Add video picker and Discord video attachments.

The picker accepts one MP4 or WebM, the thread lightbox zooms photos, and Discord gets the video as a file on a follow-up message so photo embeds still render.

internal/web/server_test.go

@@ -176,8 +176,10 @@ func TestRegisterLoginAsk(t *testing.T) {
 		`enctype="multipart/form-data"`,
 		`data-image-picker`,
 		`id="submit-images"`,
-		`accept="image/jpeg,image/png,image/webp"`,
-		`Add up to 4 JPEG, PNG, or WebP images.`,
+		`accept="image/jpeg,image/png,image/webp,video/mp4,video/webm,.mp4,.webm"`,
+		`Add up to 4 JPEG, PNG, or WebP images (5 MB each) and one MP4 or WebM video (25 MB).`,
+		`Drop photos or a video here`,
+		`data-video-slot`,
 	} {
 		if !strings.Contains(rec.Body.String(), want) {
 			t.Fatalf("submit form missing %q: %s", want, rec.Body.String())