feat(web): use the Ace editor for the request-metadata JSON

A textarea is a poor place to hand-write a nested `_meta` payload. Ace closes a
brace when you open one, folds nested objects, and — through its JSON worker —
marks the offending line in the gutter instead of reporting one message for the
whole document.

Three things the swap turned up:

- Ace fires *two* change events for a replace (a remove, then an insert), so
  select-all-and-retype briefly leaves an empty document. Acting on the first
  emitted `{}` and wiped the user's saved metadata the instant they selected
  it. `handleChange` now coalesces the pair in a microtask, and the component
  tracks what it last emitted so the parent echoing that value back is not
  mistaken for an external reset.

- Ace's own `github` themes fail WCAG AA on the gutter — `#AAA` on `#e8e8e8` is
  1.89:1, and the dark theme is 4.13:1. Folding needs the gutter, so the colors
  are overridden with palette tokens (5.82:1 and 4.88:1) rather than hiding it.

- `textInputAriaLabel` alone never reaches the DOM: Ace composes the label as
  "<label>, Cursor at row N" and only recomputes it when the cursor moves, so
  it is forced once on load. Until then the control's only accessible name is a
  position readout.

Testing is split because Ace's input path does not work under happy-dom at all
— `userEvent.type` reaches the textarea and produces no edit, so a test written
that way asserts nothing. Unit tests drive the editor instance through a shared
`src/test/aceEditor.ts` helper; the real keyboard behaviour (`{` then Enter
giving `{\n  \n}`) is asserted by Storybook play functions in Chromium.

`react-ace`/`ace-builds` are declared in `clients/web` rather than the root, per
the rule for a dependency that renders React components.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/package.json

@@ -42,6 +42,7 @@
     "@mantine/hooks": "^8.3.17",
     "@mantine/notifications": "^8.3.17",
     "@napi-rs/keyring": "^1.3.0",
+    "ace-builds": "^1.44.0",
     "ajv": "^8.17.1",
     "atomically": "^2.1.1",
     "chokidar": "^4.0.3",
@@ -51,6 +52,7 @@
     "papaparse": "^5.5.4",
     "pino": "^9.14.0",
     "react": "^19.2.4",
+    "react-ace": "^15.0.0",
     "react-dom": "^19.2.4",
     "react-icons": "^5.6.0",
     "react-markdown": "^10.1.0",