Add global keyboard EventRouter and unify publish (M6, §01D)

One module owns the document keydown listener and dispatches the shortcut map
(Cmd/Ctrl+Shift+N / +K / +S / +, / Escape), platform-aware, via the single-source
focus-utils interactive-context gate. Escape and Cmd/Ctrl+S run before the gate so
save works while editing; the rest are suppressed mid-typing. Publish + its toast
move into services/snippet-actions so the button and Cmd/Ctrl+S behave identically.
Removes the ad-hoc keydown handler from App and Monaco's Cmd+S command.

docs/spec/01-application-shell.md

@@ -67,6 +67,10 @@ Notes:
 
 - Cmd/Ctrl + K is a toggle: if the Datasets manager is already open it closes it; otherwise it opens it.
 - Escape only acts when a modal is open; with no modal open it does nothing.
+- Cmd/Ctrl + S publishes the active draft regardless of where focus is — **including
+  while the editor has focus** — so it behaves as a "save" you reach for mid-edit. The
+  other shortcuts (new snippet, toggle Datasets, settings) are suppressed while the
+  user is typing in the editor or an input, so they don't interrupt text entry.
 - The shortcut actions override the browser's default behavior for those key combinations.
 
 ## E. Navigation & Shareable URL State