fix: 4 correctifs UX — workspace +, library empty, settings spacing

- Workspaces: bouton bleu '+' maintenant centré, 48px, min-height 140px (plus coupé)
- Library no-workspace: 'Open a Workspace...' avec lien Go to Workspaces au lieu de Loading
- Library: boutons '+ Add new' et 'New page' cachés quand pas de workspace
- Settings: espacement bouton Upload photo (margin-bottom, margin-top augmentés)
- 143 tests passent

app/templates/settings.html

@@ -113,11 +113,11 @@
               <span x-show="!avatarUrl">{{ user.get('full_name', user.get('login',''))[:1].upper() }}</span>
             </div>
             <div>
-              <label class="avatar-upload-label">
+              <label class="avatar-upload-label" style="margin-bottom:4px;">
                 Upload photo
                 <input type="file" class="avatar-upload-input" accept="image/*" @change="uploadAvatar($event)">
               </label>
-              <div style="font-size:11px;color:var(--text-dim);margin-top:4px;">JPG, PNG or GIF — max 2MB</div>
+              <div style="font-size:11px;color:var(--text-dim);margin-top:8px;">JPG, PNG or GIF — max 2MB</div>
             </div>
           </div>
           <div style="margin-top:8px;">