Restore Replace shortcut and document Move Line in shortcuts modal

[NisargJasani0602]

Summary

Partial fix for #4253 (keyboard shortcuts modal out of sync with actual
keybindings). Covers two of the items identified there:

• Restored the Replace shortcut (Ctrl+H on Windows/Linux, Cmd+Alt+F on Mac — matching Sublime and the pre-CM6 config). This was a genuine regression from the CM5 -> CM6 migration, not just a documentation gap: the old editor explicitly bound this via CM5's replace command, and the modal already advertised the correct platform-specific combo, but nothing implemented it. Fixed by wiring it to CodeMirror 6's openSearchPanel, whose panel already includes Replace/Replace All fields built in.
• Added Move Line Up/Down to the shortcuts modal (from CTRL+SHIFT+UP no longer moves lines #4222, already merged, just never documented).

Scope note

While auditing the modal against the actual keymap, I found several more real gaps (a phantom "Comment Line" shortcut that isn't implemented, and several working-but-undocumented shortcuts like select-next-occurrence, multi-cursor, delete-line, go-to-line, etc).
Keeping those out of this PR to stay reviewable — tracking as a follow-up.

Test plan

• npm run lint passes
• npm test -- client/modules/IDE/components/Editor passes
• Manually verified in the running dev server: Cmd+Alt+F opens
  the search panel with Replace fields; the modal shows the new
  Move Line Up/Down entries

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines

[NisargJasani0602]

Quick note before this gets reviewed — I want to flag something I noticed after implementing this.

Ctrl+F (already bound to openSearchPanel) opens the same search panel that already includes the Replace field and buttons built in — there's no separate "replace mode" in CodeMirror 6's search UI. So the Ctrl+H/Cmd+Alt+F binding added here doesn't unlock any new functionality; it's just an additional key combo that calls the exact same openSearchPanel function Ctrl+F already calls.

I still think there's a reasonable case for keeping it (it restores what the pre-CM6 editor explicitly had, and the shortcuts modal has been advertising a dedicated Replace shortcut since before I touched anything, so this closes a documented promise), but it's also fair to call it redundant — one keystroke away either way, and the Mac binding (Cmd+Alt+F) is a little awkward specifically because Cmd+H is OS-reserved.

Two ways to go from here:

Keep this PR as-is (dedicated Replace shortcut).
Simplify instead — drop the separate shortcut, and just document Ctrl+F as "Find / Replace" in the modal, since it already covers both.
Happy to go either way — let me know your preference and I'll adjust.