Experimental changes to make table fit+scroll

- limit individual cells to 15em, and show ellipses when truncated
- limit total table width to body width, so that inner table is shown
with scrollbar
- use class rather than id - ids are bad practice in Svelte components,
as more than one may be displayed on a single page

ts/import-csv/Preview.svelte

@@ -10,7 +10,8 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     export let preview: Generic.StringList[];
 </script>
 
-<table id="preview">
+<div class="outer">
+    <table class="preview">
         {#each columnOptions.slice(1) as { label }}
             <th>
                 {label}
@@ -24,17 +25,26 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
             </tr>
         {/each}
     </table>
+</div>
 
 <style lang="scss">
-    #preview {
+    .outer {
+        // approximate size based on body max width + margins
+        width: min(90vw, 65em);
+        overflow: auto;
+    }
+
+    .preview {
         border-collapse: collapse;
-        width: 100%;
         white-space: nowrap;
 
         th,
         td {
+            text-overflow: ellipsis;
+            overflow: hidden;
             border: 1px solid var(--faint-border);
             padding: 0.25rem 0.5rem;
+            max-width: 15em;
         }
 
         th {