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/styles/sidePanel.ts

@@ -0,0 +1,9 @@
+import { toRem } from 'folds';
+
+/**
+ * One width for every right-hand room panel (members, bookmarks, threads, a
+ * thread, media gallery, widgets). Swapping one panel for another must not
+ * resize the room view: when the widths differed, the header icons slid
+ * sideways and a second click on the same spot hit a different button (#263).
+ */
+export const SIDE_PANEL_WIDTH = toRem(340);