feat(sidebar): add per-page hide toggle with more options dropdown

- Add ``` button in sidebar header with dropdown menu
- Add "Hide sidebar on this page" option with per-page localStorage state
- Show edge indicator (14px chevron) on left when hidden per-page
- Show brief toast notification when hiding
- Fix container margin not resetting when sidebar is per-page hidden
- Add i18n translations for all 10 locales

locales/en.json

@@ -953,9 +953,13 @@
     },
     "sidebar": {
         "modelRoot": "Root",
+        "moreOptions": "More options",
         "collapseAll": "Collapse All Folders",
         "pinSidebar": "Pin Sidebar",
         "unpinSidebar": "Unpin Sidebar",
+        "hideOnThisPage": "Hide sidebar on this page",
+        "showSidebar": "Show sidebar",
+        "sidebarHiddenNotification": "Folder sidebar hidden on {page} page",
         "switchToListView": "Switch to List View",
         "switchToTreeView": "Switch to Tree View",
         "recursiveOn": "Include subfolders",