fix(room): one width for every right-hand panel (#263)

Members and bookmarks were 266px, the gallery 320, the threads list 340,
a thread and widgets 360. Swapping panels resized the room view, so the
header icons slid sideways (74px for members -> threads) and a second
click on the same spot opened Widgets instead of closing the panel. All
six now use SIDE_PANEL_WIDTH (340px, styles/sidePanel.ts).

Fixes #263

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/room/widgets/WidgetsPanel.css.ts

@@ -1,8 +1,9 @@
 import { style } from '@vanilla-extract/css';
-import { config, toRem } from 'folds';
+import { config } from 'folds';
+import { SIDE_PANEL_WIDTH } from '../../../styles/sidePanel';
 
 export const WidgetsPanel = style({
-  width: toRem(360),
+  width: SIDE_PANEL_WIDTH,
   '@media': {
     '(max-width: 750px)': {
       position: 'fixed',