feat(sidebar): project notes/todos live in right sidebar context tab

Replace the header sticky-note popover/mobile overlay with a dedicated Context
tab in the right sidebar. Context is cycled by the right-sidebar shortcut
alongside git and files, and persisted across reloads.

packages/ui/src/hooks/useKeyboardShortcuts.ts

@@ -182,7 +182,7 @@ export const useKeyboardShortcuts = () => {
           return;
         }
 
-        const tabs = ['git', 'files'] as const;
+        const tabs = ['git', 'files', 'context'] as const;
         const currentIndex = tabs.indexOf(rightSidebarTab);
         const nextTab = tabs[(currentIndex + 1) % tabs.length];