feat(dashboard): CodeMirror editors for Rhai source + JSON

Replaces the four <textarea> usages with a CodeMirror 6 editor that
brings, just by being a real editor: syntax highlighting, line
numbers, bracket matching, multi-cursor, proper undo/redo, and
search/replace (Ctrl+F / Ctrl+H). Plus a Rhai-aware autocomplete and
a "Format JSON" button on the test-invoke panels.

Per discussion, deliberately did NOT add: LSP, go-to-definition,
Rhai formatter (none exists), or anything else IDE-shaped. The
existing CodeEditor component is wired so swapping the language
extension later is a one-line change.

Lay of the land (from the research pass):
  * No CodeMirror Rhai package exists on npm.
  * No Rhai formatter exists anywhere.
  * The Rhai authors publish a TextMate grammar at
    rhaiscript/vscode-rhai (MPL-2.0). We don't load the full
    grammar (would cost ~250KB of vscode-textmate + oniguruma);
    we cite it as the source-of-truth for our keyword/operator
    lists in a small custom StreamLanguage.
  * rhaiscript/lsp exists but is experimental + unmaintained
    since 2023; skipped.

Files:
  * dashboard/src/lib/editor-theme.ts — CodeMirror theme +
    HighlightStyle wired to the existing slate/sky palette so the
    editor blends into the cards instead of looking transplanted.
  * dashboard/src/lib/rhai-mode.ts — StreamLanguage tokenizer for
    Rhai with the upstream grammar's keyword/operator lists, plus
    a completion source pulling ctx.* / log::* from our SDK
    contract suite (the authoritative list).
  * dashboard/src/lib/CodeEditor.svelte — wraps EditorView with
    two-way $bindable() value, language picker ('rhai' | 'json'),
    placeholder, minHeight props. Guards against the update
    listener echoing parent-driven changes back as edits.
  * Replaces textareas in:
      routes/+page.svelte                 — create form source
      routes/scripts/[id]/+page.svelte    — Edit tab source +
                                            Test invoke body +
                                            headers
  * Format buttons next to the body/headers editors run
    JSON.stringify(JSON.parse(value), null, 2); errors surface
    inline next to the button without trashing the field.

Bundle:
  * +~430KB to the CodeMirror chunk in dashboard build (~150KB
    gzipped on the wire). Lazy-loaded — only fetched when a route
    that uses CodeEditor renders.
  * `npm install` clean, 0 vulnerabilities, `npm run check`
    clean, `npm run build` clean.

No backend / API / SDK / schema / wire changes. No version bumps.

dashboard/package.json

@@ -15,9 +15,9 @@
   "devDependencies": {
     "@eslint/js": "^9.18.0",
     "@sveltejs/adapter-static": "^3.0.8",
-    "@types/node": "^22.10.5",
     "@sveltejs/kit": "^2.17.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
+    "@types/node": "^22.10.5",
     "eslint": "^9.18.0",
     "eslint-config-prettier": "^10.0.1",
     "eslint-plugin-svelte": "^3.0.0",
@@ -32,5 +32,16 @@
   },
   "overrides": {
     "cookie": "^0.7.2"
+  },
+  "dependencies": {
+    "@codemirror/autocomplete": "^6.20.2",
+    "@codemirror/commands": "^6.10.3",
+    "@codemirror/lang-json": "^6.0.2",
+    "@codemirror/language": "^6.12.3",
+    "@codemirror/search": "^6.7.0",
+    "@codemirror/state": "^6.6.0",
+    "@codemirror/view": "^6.43.0",
+    "@lezer/highlight": "^1.2.3",
+    "codemirror": "^6.0.2"
   }
 }