perf(json-editor): unify admin JSON editing experience (#6421)

* perf(json-editor): improve JSON editing experience

- integrate Yace for syntax highlighting, history, indentation, auto-closing, and smart line breaks.
- add copy support, cursor location feedback, and synchronized content and line-number scrolling.
- extract JSON editor utilities and cover key interactions with unit tests.

* perf(system-settings): improve JSON configuration editing

- replace raw JSON textareas with the shared editor for highlighting, validation, copy, and formatting.
- preserve field-specific examples and make placeholders visible through the transparent editor layer.
- remove duplicate formatting controls while keeping existing form validation and save behavior.

* perf(json-editor): standardize JSON inputs across admin settings

- replace pure JSON textareas with the shared editor across system settings and channel workflows.
- preserve form focus, validation, placeholders, and visual or JSON editing modes.
- add happy-dom component coverage for form bindings, controlled updates, and formatting.

* fix(json-code-editor): address accessibility review findings

- Drop the unconditional aria-label that overrode every field's
  label-derived accessible name; add an optional ariaLabel prop and
  set it at call sites without an associated label
- Associate standalone Labels via htmlFor/id in channel-affinity views
- Hide the highlight mirror and line-number layers from the
  accessibility tree (aria-hidden)
- Give the line-number gutter an opaque background so horizontally
  scrolled code no longer slides under it
- Degrade to no scroll sync instead of destroying the editor when the
  line-number layer is not found

web/package.json

@@ -74,6 +74,7 @@
     "tw-animate-css": "^1.4.0",
     "use-stick-to-bottom": "^1.1.6",
     "vaul": "^1.1.2",
+    "yace": "^1.1.0",
     "zod": "^4.4.3",
     "zustand": "^5.0.14"
   },
@@ -90,6 +91,7 @@
     "@typescript/native-preview": "^7.0.0-dev.20260702.3",
     "@xyflow/react": "^12.11.1",
     "embla-carousel-react": "^8.6.0",
+    "happy-dom": "^20.11.1",
     "knip": "^6.24.0",
     "oxfmt": "^0.57.0",
     "oxlint": "^1.72.0",