Add contact antispam and fix gallery video playback.

English-only messages, rate limiting, min fill time, and normalized email
validation; improve modal video serving with posters, correct MIME types, and
no gzip on gallery media.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/gallery.templ

@@ -65,7 +65,15 @@ templ ImageModal(img gallery.Image, prevPath, nextPath string) {
 			</button>
 			<figure class="modal-figure">
 				if img.IsVideo {
-					<video src={ img.URL } controls playsinline preload="metadata"></video>
+					<div class="modal-video">
+						<video controls playsinline preload="auto" poster={ img.ThumbURL }>
+							<source src={ img.URL } type={ videoMIME(img.Filename) }/>
+						</video>
+						<p class="video-unavailable" hidden>
+							Playback is not supported in this browser.
+							<a href={ img.URL } download={ img.Filename }>Download the video</a>
+						</p>
+					</div>
 				} else {
 					<img src={ img.URL } alt={ img.Filename }/>
 				}