feat: add spinner to post button during gallery creation

src/components/atoms/grain-spinner.js

···
 import { LitElement, html, css } from 'lit';
 
 export class GrainSpinner extends LitElement {
+  static properties = {
+    size: { type: Number }
+  };
+
   static styles = css`
     :host {
       display: flex;
       justify-content: center;
       align-items: center;
+    }
+    :host(:not([size])) {
       width: 100%;
       padding: var(--space-lg) 0;
     }
     .spinner {
       display: block;
-      width: 24px;
-      height: 24px;
       border: 2px solid var(--color-bg-elevated);
       border-top-color: var(--color-text-secondary);
       border-radius: 50%;
···
     }
   `;
 
+  constructor() {
+    super();
+    this.size = null;
+  }
+
   render() {
-    return html`<div class="spinner"></div>`;
+    const size = this.size || 24;
+    return html`<div class="spinner" style="width: ${size}px; height: ${size}px;"></div>`;
   }
 }
 

src/components/pages/grain-create-gallery.js

···
 import { auth } from '../../services/auth.js';
 import { draftGallery } from '../../services/draft-gallery.js';
 import '../atoms/grain-icon.js';
+import '../atoms/grain-spinner.js';
 
 const UPLOAD_BLOB_MUTATION = `
   mutation UploadBlob($data: String!, $mimeType: String!) {
···
           ?disabled=${!this.#canPost}
           @click=${this.#handlePost}
         >
-          ${this._posting ? 'Posting...' : 'Post'}
+          ${this._posting ? html`<grain-spinner size="16"></grain-spinner>` : 'Post'}
         </button>
       </div>