feat(app): add focus mode (Cmd+Shift+F) to show only the active pane

Hides ScreenHeader, LeftSidebar, and ExplorerSidebar on desktop,
replacing the split layout with the single focused pane. Respects
macOS traffic light padding in the tab bar when active. Shortcut
is restricted to workspace screens. Also nudges traffic light
buttons up 3px globally.

packages/app/src/keyboard/actions.ts

@@ -41,6 +41,7 @@ export type KeyboardActionId =
   | "workspace.terminal.new"
   | "worktree.new"
   | "worktree.archive"
+  | "view.toggle.focus"
   | "message-input.action";
 
 export type KeyboardShortcutPayload =

packages/app/src/keyboard/keyboard-shortcuts.ts

@@ -718,6 +718,32 @@ const SHORTCUT_BINDINGS: readonly ShortcutBinding[] = [
     },
   },
 
+  // --- Focus mode ---
+  {
+    id: "view-toggle-focus-cmd-shift-f-mac",
+    action: "view.toggle.focus",
+    combo: "Cmd+Shift+F",
+    when: { mac: true, hasSelectedAgent: true, commandCenter: false },
+    help: {
+      id: "toggle-focus",
+      section: "panels",
+      label: "Toggle focus mode",
+      keys: ["mod", "shift", "F"],
+    },
+  },
+  {
+    id: "view-toggle-focus-ctrl-shift-f-non-mac",
+    action: "view.toggle.focus",
+    combo: "Ctrl+Shift+F",
+    when: { mac: false, hasSelectedAgent: true, commandCenter: false, terminal: false },
+    help: {
+      id: "toggle-focus",
+      section: "panels",
+      label: "Toggle focus mode",
+      keys: ["mod", "shift", "F"],
+    },
+  },
+
   // --- Message input ---
   {
     id: "message-input-voice-toggle-cmd-shift-d-mac",