feat: hidden flag, description placeholders, drag handle contrast fix

- Unit and Group get hidden?: boolean — filtered server-side before render
- Hidden group also suppresses all its units from the converter
- New resolveDescription() utility in src/lib/description.ts
  supports {id}, {id:label}, {id:plural}, {id:symbol} placeholders
- ConverterCard pre-resolves descriptions and passes result to ConversionResult
- ConversionResult accepts description prop instead of reading unit.description
- Admin forms: Hidden checkbox added for both units and groups
- drag-handle color overridden to --pico-primary-inverse on selected/drag-over rows
  (white in light/dark themes, black on Dan Mode yellow)

src/routes/admin/+page.svelte

@@ -861,6 +861,10 @@
         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>
+      <label class="checkbox-label">
+        <input type="checkbox" bind:checked={unitForm.hidden} />
+        Hidden <small class="muted">— exclude from converter rendering</small>
+      </label>
       <div class="form-actions">
         <button onclick={saveUnit}>{unitFormNew ? 'Create' : 'Save'}</button>
         {#if !unitFormNew}
@@ -906,6 +910,10 @@
           <option value="alpha">Alphabetical</option>
         </select>
       </label>
+      <label class="checkbox-label">
+        <input type="checkbox" bind:checked={groupForm.hidden} />
+        Hidden <small class="muted">— exclude group and all its units from converter rendering</small>
+      </label>
       <div class="form-actions">
         <button onclick={saveGroup}>{groupFormNew ? 'Create' : 'Save'}</button>
         {#if !groupFormNew}
@@ -1219,6 +1227,19 @@
     gap: 0.5rem;
     margin-top: 1rem;
   }
+  .checkbox-label {
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+    cursor: pointer;
+    margin-bottom: 0.75rem;
+  }
+  .checkbox-label input[type=checkbox] {
+    flex-shrink: 0;
+    width: 1rem;
+    height: 1rem;
+    margin: 0;
+  }
   .form-msg {
     padding: 0.5rem 0.75rem;
     border-radius: var(--pico-border-radius);
@@ -1289,6 +1310,12 @@
   .drag-handle:active {
     cursor: grabbing;
   }
+  /* When the row has a primary-colored background, muted grey/orange disappears.
+     Force the handle to the primary inverse (white in normal themes, black in Dan). */
+  .tree-unit-row.selected .drag-handle,
+  .tree-unit-row.drag-over-unit .drag-handle {
+    color: var(--pico-primary-inverse);
+  }
   .drag-over-unit {
     border-top: 2px solid var(--pico-primary);
   }