feat: unit descriptions with tooltip action and admin textarea

src/routes/admin/+page.svelte

@@ -464,6 +464,10 @@
           <small class="tobase-hint">{toBaseHint}</small>
         {/if}
       </label>
+      <label>
+        Description <small class="muted">(optional — shown as tooltip in converter)</small>
+        <textarea bind:value={unitForm.description} placeholder="e.g. A unit based on…" rows="2"></textarea>
+      </label>
       <div class="form-actions">
         <button onclick={saveUnit}>{unitFormNew ? 'Create' : 'Save'}</button>
         {#if !unitFormNew}