Keyboard shortcuts modal is out of sync with actual keybindings

Follow-up to #4222 (see this comment for context) — KeyboardShortcutModal.jsx has drifted from what's actually wired up in keymaps.js. A few known discrepancies:

Advertised but not implemented:

• The modal lists {Ctrl/Cmd}+H for "Replace," but there's no replace command wired up anywhere in keymaps.js. Worth deciding: implement it (CodeMirror 6's search panel already supports replace, just needs to be exposed on this shortcut), or remove the claim from the modal if it's not planned.

Implemented but not advertised:

• Ctrl-Shift-Up/Down (Cmd-Ctrl-Up/Down on Mac) — move line up/down, added in #4250.
• Shift-Ctrl-D (Shift-Cmd-D on Mac) — duplicate line, and Ctrl-Up/Down (Ctrl-Alt-Up/Down on Mac) — scroll without moving cursor, both pending in a follow-up PR.

Proposal

1. Decide the fate of the Replace shortcut (implement vs. remove from modal).
2. Add entries to KeyboardShortcutModal.jsx for the shortcuts above.

Happy to take this on.

Start by comparing KeyboardShortcutModal.jsx with keymaps.js, then review the context in #4222 and the implementation from #4250. Resolve whether the Replace shortcut should be implemented or removed, and document the listed implemented or pending shortcuts in the modal. Done means the modal matches the keybindings that are actually wired up.