[web] Add a focus mode to the editor (#32245)

* Basic focus mode implementation

* Exit button

* Toggle between editor pdf mode

* Keep the rail and file tree mounted for editor tabs to work properly

* Hide Switch to editor button in focus mode

* Show shortcuts in layout menu

* Gate the feature behind a focus-mode feature flag

* Add split test badges in the focus mode dropdown item and top bar

* Add analytics tracking for focusMode changes

* Cleanup leftover IdeView value

* Allow passing focusMode value in layout test provider

* Remove redundant listener for focus mode shortcut

* Update focus mode and full project search listeners to use the event key

* Post-rebase cleanup

---------

Co-authored-by: Anna Fields <acfields11@gmail.com>
GitOrigin-RevId: 207f8d136fa6662558c474cca4a867a9842191e7

services/web/locales/en.json

@@ -849,6 +849,7 @@
   "examples_to_help_you_learn": "Examples to help you learn how to use powerful LaTeX packages and techniques.",
   "exclusive_access_with_labs": "Exclusive access to early-stage experiments",
   "existing_plan_active_until_term_end": "Your existing plan and its features will remain active until the end of the current billing period.",
+  "exit_focus_mode": "Exit focus mode",
   "expand": "Expand",
   "experiment_enabled_refresh_page": "Experiment is enabled! You need to refresh the page to see the changes.",
   "experiment_full_check_back_soon": "Sorry, this experiment is full. Spaces may become available, so check back soon.",
@@ -927,6 +928,7 @@
   "fixed_width_wrap_text": "Fixed width, wrap text",
   "flexible_plans_for_everyone": "Flexible plans for everyone—from individual students and researchers, to large businesses and universities.",
   "flexible_plans_for_everyone_from_individual_students_and_researchers_to_large_businesses_and_universities": "Flexible plans for everyone, from individual students and researchers to large businesses and universities.",
+  "focus_mode": "Focus mode",
   "fold_line": "Fold line",
   "folder_location": "Folder location",
   "folder_name": "Folder name",