Frontend: don't show img preview for video/audio/doc files

- MediaPreview.vue: only render <img> for non-video, non-audio items
  (video/audio already have their own player containers)
- Document.ts: limit previewurl to previewable files (was checking only .complete)

frontend/src/components/MediaPreview.vue

@@ -2,7 +2,7 @@
   <div v-if=showProgress() class="preview-progress" aria-label="Preview pending">
     <SpinnerIcon />
   </div>
-  <img v-else-if=previewSrc :src="previewSrc" alt="">
+  <img v-else-if="previewSrc && !video() && !audio()" :src="previewSrc" alt="">
   <img v-else-if=doc.img :src=doc.url alt="">
   <span v-else-if=doc.dir class="folder icon"></span>
   <div v-else-if=video() class="video-container" :class="{ pending: !doc.complete }">

frontend/src/repositories/Document.ts

@@ -70,7 +70,7 @@ export class Doc {
     return ['mp4', 'mkv', 'webm', 'ogg', 'mp3', 'flac', 'aac', 'pdf'].includes(this.ext)
   }
   get previewurl(): string {
-    if (!this.complete || this.dir) return ''
+    if (!this.complete || !this.previewable) return ''
     return this.url.replace(/^\/files/, '/preview')
   }
   get ext(): string {