feat(editor): Mermaid syntax highlighting, linter and autocomplete

- StreamLanguage tokenizer for Mermaid: diagram-type keywords, general
  keywords, directions, arrows/links, strings, %% comments, init
  directives and YAML frontmatter, with a dark-theme HighlightStyle
- live linter via mermaid.parse: maps parse errors to the right editor
  line (accounting for the frontmatter lines mermaid strips), with wavy
  underline, gutter marker and message tooltip
- autocomplete: Mermaid keyword/type/direction options plus starter
  diagram snippets offered on the first line; bracket closing
- dark-themed completion + diagnostic tooltips

Adds @codemirror/lint, @codemirror/autocomplete and @lezer/highlight.

src/lib/components/Editor.svelte

@@ -17,7 +17,10 @@
     indentWithTab,
   } from '@codemirror/commands';
   import { bracketMatching, indentOnInput } from '@codemirror/language';
+  import { closeBrackets, closeBracketsKeymap } from '@codemirror/autocomplete';
+  import { lintKeymap } from '@codemirror/lint';
   import { store } from '../store.svelte';
+  import { mermaidSupport } from '../cm-mermaid';
 
   let host: HTMLDivElement;
   let view: EditorView | undefined;
@@ -48,6 +51,43 @@
         backgroundColor: 'var(--accent-soft) !important',
       },
       '.cm-matchingBracket': { backgroundColor: 'var(--accent-soft)', outline: 'none' },
+
+      // Autocomplete popup
+      '.cm-tooltip': {
+        backgroundColor: 'var(--bg-elev-2)',
+        border: '1px solid var(--border-strong)',
+        borderRadius: '6px',
+        color: 'var(--text)',
+        boxShadow: '0 8px 28px rgba(0,0,0,0.45)',
+      },
+      '.cm-tooltip.cm-tooltip-autocomplete > ul': { fontFamily: 'var(--mono)', maxHeight: '16em' },
+      '.cm-tooltip-autocomplete ul li[aria-selected]': {
+        backgroundColor: 'var(--accent)',
+        color: '#fff',
+      },
+      '.cm-completionIcon': { color: 'var(--text-dim)', paddingRight: '0.6em' },
+      '.cm-completionDetail': { color: 'var(--text-faint)', fontStyle: 'normal' },
+      '.cm-completionInfo': {
+        backgroundColor: 'var(--bg-elev-2)',
+        border: '1px solid var(--border-strong)',
+        borderRadius: '6px',
+        color: 'var(--text-dim)',
+      },
+
+      // Lint diagnostics
+      '.cm-diagnostic': {
+        fontFamily: 'var(--mono)',
+        fontSize: '12px',
+        borderLeft: '3px solid var(--red)',
+        padding: '6px 8px',
+      },
+      '.cm-diagnostic-error': { borderLeftColor: 'var(--red)' },
+      '.cm-lintRange-error': {
+        backgroundImage: 'none',
+        textDecoration: 'underline wavy var(--red)',
+        textDecorationSkipInk: 'none',
+      },
+      '.cm-lint-marker-error': { content: '""' },
     },
     { dark: true }
   );
@@ -65,8 +105,16 @@
           drawSelection(),
           indentOnInput(),
           bracketMatching(),
+          closeBrackets(),
           highlightActiveLine(),
-          keymap.of([...defaultKeymap, ...historyKeymap, indentWithTab]),
+          ...mermaidSupport(),
+          keymap.of([
+            ...closeBracketsKeymap,
+            ...defaultKeymap,
+            ...historyKeymap,
+            ...lintKeymap,
+            indentWithTab,
+          ]),
           theme,
           EditorView.lineWrapping,
           EditorView.updateListener.of((u) => {