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/src/lib/editor-theme.ts

@@ -0,0 +1,148 @@
+// Dashboard-matching theme + highlight style for CodeMirror.
+//
+// Colors are pulled from the existing slate/sky palette used across
+// the dashboard (#0f172a / #1e293b / #38bdf8) so the editor blends
+// into the surrounding cards instead of looking like a third-party
+// transplant. Stock dark themes like "One Dark" or "VS Code Dark"
+// would each clash with the slate background by a few shades.
+
+import { EditorView } from '@codemirror/view';
+import { HighlightStyle, syntaxHighlighting } from '@codemirror/language';
+import { tags as t } from '@lezer/highlight';
+
+const palette = {
+	bg: '#0b1220', // matches existing <input>/<textarea> backgrounds
+	bgGutter: '#0f172a',
+	border: '#334155',
+	text: '#e2e8f0',
+	textMuted: '#94a3b8',
+	cursor: '#38bdf8',
+	selection: '#38bdf830',
+	activeLine: '#1e293b',
+	matchingBracket: '#38bdf850',
+	searchMatch: '#38bdf850',
+	searchMatchSelected: '#38bdf8',
+
+	// Syntax — chosen to be readable against #0b1220 without making
+	// any single token feel louder than the rest.
+	comment: '#64748b',
+	str: '#86efac',
+	num: '#fbbf24',
+	bool: '#fbbf24',
+	keyword: '#c4b5fd',
+	control: '#c4b5fd',
+	operator: '#cbd5e1',
+	punct: '#cbd5e1',
+	function: '#38bdf8',
+	property: '#e2e8f0',
+	variable: '#e2e8f0',
+	ctx: '#f472b6', // `ctx` is special — pinker so users notice it
+	namespace: '#fbbf24', // `log::`
+	invalid: '#ef4444'
+};
+
+export const dashboardTheme = EditorView.theme(
+	{
+		'&': {
+			color: palette.text,
+			backgroundColor: palette.bg,
+			borderRadius: '0.375rem'
+		},
+		'&.cm-focused': {
+			outline: `1px solid ${palette.cursor}`
+		},
+		'.cm-content': {
+			caretColor: palette.cursor,
+			fontFamily:
+				'ui-monospace, SFMono-Regular, Menlo, Consolas, "Liberation Mono", monospace',
+			fontSize: '0.85rem',
+			padding: '0.5rem 0'
+		},
+		'.cm-cursor, .cm-dropCursor': {
+			borderLeftColor: palette.cursor,
+			borderLeftWidth: '2px'
+		},
+		'&.cm-focused .cm-selectionBackground, .cm-selectionBackground, .cm-content ::selection':
+			{
+				backgroundColor: palette.selection
+			},
+		'.cm-activeLine': {
+			backgroundColor: palette.activeLine
+		},
+		'.cm-gutters': {
+			backgroundColor: palette.bgGutter,
+			color: palette.textMuted,
+			border: 'none',
+			borderRight: `1px solid ${palette.border}`
+		},
+		'.cm-activeLineGutter': {
+			backgroundColor: palette.activeLine,
+			color: palette.text
+		},
+		'.cm-matchingBracket, .cm-nonmatchingBracket': {
+			backgroundColor: palette.matchingBracket,
+			outline: 'none'
+		},
+		// Search / replace panel (Ctrl+F)
+		'.cm-panels': {
+			backgroundColor: palette.bgGutter,
+			color: palette.text,
+			borderTop: `1px solid ${palette.border}`
+		},
+		'.cm-searchMatch': {
+			backgroundColor: palette.searchMatch
+		},
+		'.cm-searchMatch.cm-searchMatch-selected': {
+			backgroundColor: palette.searchMatchSelected
+		},
+		'.cm-panel input, .cm-panel button': {
+			backgroundColor: palette.bg,
+			color: palette.text,
+			border: `1px solid ${palette.border}`,
+			borderRadius: '0.25rem',
+			padding: '0.2rem 0.4rem'
+		},
+		'.cm-panel button:hover': {
+			backgroundColor: palette.activeLine
+		},
+		// Autocomplete popup
+		'.cm-tooltip': {
+			backgroundColor: palette.bgGutter,
+			border: `1px solid ${palette.border}`,
+			color: palette.text
+		},
+		'.cm-tooltip.cm-tooltip-autocomplete > ul > li[aria-selected]': {
+			backgroundColor: palette.activeLine,
+			color: palette.text
+		},
+		'.cm-completionLabel': {
+			color: palette.text
+		},
+		'.cm-completionDetail': {
+			color: palette.textMuted,
+			fontStyle: 'normal'
+		},
+		'.cm-completionIcon': {
+			color: palette.textMuted
+		}
+	},
+	{ dark: true }
+);
+
+export const dashboardHighlight = HighlightStyle.define([
+	{ tag: t.comment, color: palette.comment, fontStyle: 'italic' },
+	{ tag: [t.string, t.special(t.string)], color: palette.str },
+	{ tag: [t.number, t.bool, t.null], color: palette.num },
+	{ tag: [t.keyword, t.modifier], color: palette.keyword, fontWeight: '600' },
+	{ tag: t.controlKeyword, color: palette.control, fontWeight: '600' },
+	{ tag: [t.operator, t.derefOperator, t.logicOperator], color: palette.operator },
+	{ tag: [t.punctuation, t.bracket, t.brace, t.paren, t.squareBracket], color: palette.punct },
+	{ tag: [t.function(t.variableName), t.function(t.propertyName)], color: palette.function },
+	{ tag: [t.propertyName, t.attributeName], color: palette.property },
+	{ tag: t.variableName, color: palette.variable },
+	{ tag: t.special(t.variableName), color: palette.ctx, fontWeight: '600' },
+	{ tag: t.namespace, color: palette.namespace, fontWeight: '600' },
+	{ tag: t.invalid, color: palette.invalid, textDecoration: 'underline wavy' }
+]);
+
+export const dashboardSyntaxHighlighting = syntaxHighlighting(dashboardHighlight);