feat: vim bindings in the input via CodeMirror 6

- Replace the plain <textarea> with a CodeMirror 6 EditorView that
  has @replit/codemirror-vim attached. Loaded from esm.sh, so no
  build step — finrod stays no-bundler.
- vim() extension is conditionally omitted on coarse-pointer devices
  (phones, tablets) where modal editing without a hardware Esc key
  is unusable and IME composition fights the keymap.
- Ctrl-/Cmd-Enter shortcut and the typing-debounce that refreshes
  the "Hot:" indicator both migrate to CodeMirror's keymap and
  updateListener APIs respectively.
- Editor styled to match the existing .output pane.

CHANGELOG: note under [Unreleased].

CHANGELOG.md

@@ -7,6 +7,9 @@ Versioning: [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+- Vim bindings in the input via CodeMirror 6 + `@replit/codemirror-vim` (loaded from esm.sh, no build step). Supports `i`/`Esc`, `hjkl`, word/line motions, `dd`/`yy`/`p`, undo/redo, `/` search, `:` command line. Auto-disabled on touch devices (detected via `(pointer: coarse)`) where modal editing fights the virtual keyboard.
+
 ## [0.1.0] - 2026-06-09
 
 First release. Stateless translator UI that proxies streaming requests to