fix: add explicit Join button to sidebar add-network form

The form had no submit button so there was no visible way to confirm.
Enter should have worked but was unreliable with two inputs. Added a
disabled-until-valid Join button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/components/Sidebar.tsx

@@ -157,6 +157,9 @@ export function Sidebar() {
                 style={{ fontSize: '0.75rem', marginTop: 4 }}
               />
             )}
+            <button type="submit" disabled={!newNetName.trim()} style={{ marginTop: 4, width: '100%', fontSize: '12px', padding: '3px 8px' }}>
+              Join
+            </button>
           </form>
         )}
       </div>