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-lock.json

@@ -16,6 +16,7 @@
         "@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",
@@ -25,6 +26,7 @@
         "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",
@@ -4383,6 +4385,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/ace-builds": {
+      "version": "1.44.0",
+      "resolved": "https://registry.npmjs.org/ace-builds/-/ace-builds-1.44.0.tgz",
+      "integrity": "sha512-PFNMSYqFdEUkul2Ntud0HvA09AgY+F1ag0UYdpMH60wNI/qOA8cB8tlTgoALMEwIdUPJK2CjrIQ7OnbiSS/ugQ==",
+      "license": "BSD-3-Clause"
+    },
     "node_modules/acorn": {
       "version": "8.18.0",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.18.0.tgz",
@@ -5061,6 +5069,12 @@
         "url": "https://github.com/sponsors/wooorm"
       }
     },
+    "node_modules/diff-match-patch": {
+      "version": "1.0.5",
+      "resolved": "https://registry.npmjs.org/diff-match-patch/-/diff-match-patch-1.0.5.tgz",
+      "integrity": "sha512-IayShXAgj/QMXgB0IWmKx+rOPuGMhqm5w6jvFxmVenXKIzRqTAAsbBPT3kWQeGANj3jGgvcvv4yK6SxqYmikgw==",
+      "license": "Apache-2.0"
+    },
     "node_modules/doctrine": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/doctrine/-/doctrine-3.0.0.tgz",
@@ -5487,6 +5501,15 @@
       "integrity": "sha512-f3qQ9oQy9j2AhBe/H9VC91wLmKBCCU/gDOnKNAYG5hswO7BLKj09Hc5HYNz9cGI++xlpDCIgDaitVs03ATR84Q==",
       "license": "MIT"
     },
+    "node_modules/fast-equals": {
+      "version": "5.4.1",
+      "resolved": "https://registry.npmjs.org/fast-equals/-/fast-equals-5.4.1.tgz",
+      "integrity": "sha512-DjlFSM5Pk9cGcL0q5QXl66eGzx0N6szNgaswwc5ZphlBohjTVJSnGgI+rJVOgOi65qUoQnDZN4nDqi33udtydQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/fast-json-stable-stringify": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/fast-json-stable-stringify/-/fast-json-stable-stringify-2.1.0.tgz",
@@ -8294,6 +8317,22 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/react-ace": {
+      "version": "15.0.0",
+      "resolved": "https://registry.npmjs.org/react-ace/-/react-ace-15.0.0.tgz",
+      "integrity": "sha512-gdmS5ftF0hsbkcrAjZQqYbXwFg5JrHuxjki8mP6Bn9kfa6lHKfZk9sU9EfS0ifQ1CpGCmRxF/VC7GRvlJMBuZw==",
+      "license": "MIT",
+      "dependencies": {
+        "ace-builds": "^1.36.3",
+        "diff-match-patch": "^1.0.5",
+        "fast-equals": "^5.3.3",
+        "prop-types": "^15.8.1"
+      },
+      "peerDependencies": {
+        "react": "^0.13.0 || ^0.14.0 || ^15.0.1 || ^16.0.0 || ^17.0.0 || ^18.0.0 || ^19.0.0",
+        "react-dom": "^0.13.0 || ^0.14.0 || ^15.0.1 || ^16.0.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
     "node_modules/react-docgen": {
       "version": "8.0.3",
       "resolved": "https://registry.npmjs.org/react-docgen/-/react-docgen-8.0.3.tgz",