[ZhuchkaTriplesix]

Closes #1503

main_screen.dart went from 1773 to 390 lines. No behaviour change; each step was checked with the main_screen, e2e, guards and app tests locally.

new file	what moved
main_content_split.dart	the resizable sidebar split with its spring, and the connections panel slot (were private classes of the same file)
main_screen_shortcuts.dart	the window-wide shortcut table: MainScreenShortcuts.resolve(event, cmdOrCtrl, shift, alt) returns a MainShortcut (pure, Russian layout included); the screen only maps it to an action
main_screen_navigator.dart	every pick in the tree and quick switcher (Postgres, MySQL, SQLite, extension, Redis, Mongo), the unsaved-work check, go home / close workspace
main_screen_dialogs.dart	new connection (form, URL, SQLite file), welcome tour, demo playground, files opened from the file manager, the read-only lock with its Safe Mode confirmation and re-lock timer
main_screen_chrome.dart	ShellTitleBar and ShellStatusBar as widgets

MainContentSplitState lost its underscore with the move; the perf bench looks it up by name and is updated.

Tests added: the shortcut table per key, modifier and layout; the navigator (repeat pick is a no-op, the pending Mongo collection and its notification, Redis, close vs home). The existing safe-mode, sidebar, file-launch and e2e shell tests are unchanged and pass.

One thing I left alone: the build still has a CallbackShortcuts map that repeats most of what the global handler does; removing it is a behaviour question, not part of this split.