feat(ui): redesign the default shortcut layout around a mod+k leader

Single chords stay for everyday actions; open/go actions move to two-step
mod+k sequences; held mod+digit switches header session tabs and held
mod+alt+digit switches context panel surfaces. Rare actions leave the
shortcut schema for the command palette, every remaining action ships with
a default binding, and stored overrides from the old layout reset once.
Key matching now follows the physical key on non-Latin layouts and for
Option-modified digits on macOS, including in the recording dialog.

packages/ui/tests/ego-lite/shortcut-registry.checklist.yaml

@@ -172,12 +172,12 @@ checks:
     preconditions:
       - Open a draft session with project and worktree selectors mounted.
     steps:
-      - Trigger Mod + S, P and verify the project picker opens.
+      - Trigger Mod + K, P and verify the project picker opens.
       - Press Escape once and verify it closes.
-      - Trigger Mod + S, G and verify the worktree picker opens.
+      - Trigger Mod + K, G and verify the worktree picker opens.
       - Press Escape once and verify it closes.
     assertions:
-      - A contextual Mod + S owner yields when its context is inactive.
+      - The Mod + K leader arms without a visible menu and completes on the second key.
       - Each sequence opens only its target picker.
       - One non-IME Escape closes either controlled picker.
     evidence: