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}

src/routes/admin/api/units/+server.ts

@@ -16,7 +16,7 @@ export const POST: RequestHandler = async ({ request }) => {
   const body = await request.json().catch(() => null);
   if (!body) return json({ error: 'Invalid JSON' }, { status: 400 });
 
-  const { id, label, labelPlural, symbol, group, toBase } = body as Partial<Unit>;
+  const { id, label, labelPlural, symbol, group, toBase, description } = body as Partial<Unit>;
 
   // Validation
   if (!id || !KEBAB_RE.test(id)) {
@@ -44,7 +44,8 @@ export const POST: RequestHandler = async ({ request }) => {
     labelPlural,
     symbol,
     group: group ?? null,
-    toBase
+    toBase,
+    ...(description ? { description } : {})
   };
   data.units.push(newUnit);
   saveData(data);