fix: rework compact nav and centralized back routing

Use master-detail layout on skinny windows so server and channel lists share one screen, with back returning to the space list instead of redirect loops. Add useBackRoute for title-bar and page-header back buttons with view transitions.

src/app/hooks/useCompactNav.ts

@@ -0,0 +1,26 @@
+import { useMatch } from 'react-router-dom';
+import { DIRECT_PATH, EXPLORE_PATH, HOME_PATH, INBOX_PATH, SPACE_PATH } from '../pages/paths';
+import { ScreenSize, useScreenSizeContext } from './useScreenSize';
+
+/** Skinny window layout (≤750px): master-detail instead of 3-column desktop. */
+export const useCompactNav = (): boolean => {
+  const screenSize = useScreenSizeContext();
+  return screenSize === ScreenSize.Mobile;
+};
+
+/** Routes where sidebar + channel list are shown together (no room/content pane). */
+export const useIsCompactListRoute = (): boolean => {
+  const homeMatch = useMatch({ path: HOME_PATH, caseSensitive: true, end: true });
+  const directMatch = useMatch({ path: DIRECT_PATH, caseSensitive: true, end: true });
+  const spaceMatch = useMatch({ path: SPACE_PATH, caseSensitive: true, end: true });
+  const exploreMatch = useMatch({ path: EXPLORE_PATH, caseSensitive: true, end: true });
+  const inboxMatch = useMatch({ path: INBOX_PATH, caseSensitive: true, end: true });
+
+  return !!(homeMatch || directMatch || spaceMatch || exploreMatch || inboxMatch);
+};
+
+export const useShowCompactMasterView = (): boolean => {
+  const compact = useCompactNav();
+  const isListRoute = useIsCompactListRoute();
+  return compact && isListRoute;
+};