Notebooks: add Shift+Enter run shortcut for all cells

[madesroches]

Context

#1522 / #1526 gave every cell type a Run control wired to Ctrl/Cmd+Enter (via SyntaxEditor's onRunShortcut prop, already used by every query-backed cell editor, now including markdown). There is no Shift+Enter binding anywhere today, at either the per-editor (SyntaxEditor) or notebook-wide (useNotebookKeyboardNav.ts) layer.

This was left as an open question in that plan rather than folded into the PR, since it's a scope decision that applies to every cell type, not just markdown.

Proposal

Add Shift+Enter as a run shortcut for all cells, matching Jupyter. Two possible scopes, from smallest to largest:

1. Alias only — Shift+Enter triggers the same in-place run as Ctrl/Cmd+Enter does today. Small addition to SyntaxEditor.
2. Full Jupyter parity ("run and advance") — in Jupyter, Shift+Enter runs the cell and moves focus/selection to the next cell (creating one if it's the last), while Ctrl+Enter runs in place. That "advance" behavior doesn't exist anywhere in this codebase yet and would need new plumbing through useNotebookKeyboardNav.ts to move cell focus after a run completes.

Open question

Which scope is wanted — plain alias, or full run-and-advance semantics?