feat(editor): Rainbow brackets and semantic highlighting (rainbow variables) (#59)

Two editor toggles under Settings › Editor › Color Scheme › Language Defaults (off by default, also actions `editor.toggleRainbowBrackets` / `editor.toggleSemanticHighlighting`):
- Semantic highlighting (IDEA's rainbow variables): locals and parameters coloured by a name hash from LSP semantic tokens; fields, statics, constants, library symbols untouched.
- Rainbow brackets (VS Code's bracket pair colorization): (), [], {} by depth, unmatched in the error colour, strings/comments skipped, visible ranges only with 4 KB checkpoints.
Palettes are theme tokens (`--syn-rainbow-1..5`, `--syn-bracket-1..3`, light/dark/high-contrast), editable in the Color Scheme tree, and mapped from IntelliJ `.icls` RAINBOW_COLOR1..5.

App verification on macOS found the colours were overridden by the syntax highlighter's span nested inside the marks; the rules now cover descendants. Verified both toggles on/off live in a TS file.

Fixes #56

https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/features/settings/pages/editor/colorScheme.tsx

@@ -354,9 +354,27 @@ export const COLOR_SCHEME_PAGE = editorPage({
     "background",
     "bold",
     "italic",
+    "rainbow",
     ...SCHEMES.map((s) => s.name),
   ],
   sections: [
     { items: [{ t: "custom", label: "Scheme", keywords: ATTRIBUTES.map((a) => a.name), component: ColorSchemePage }] },
+    {
+      title: "Language Defaults",
+      items: [
+        {
+          t: "check",
+          key: "editor.semanticHighlighting",
+          label: "Semantic highlighting",
+          hint: "Local variables and parameters get a colour derived from their name (needs a language server).",
+        },
+        {
+          t: "check",
+          key: "editor.rainbowBrackets",
+          label: "Rainbow brackets",
+          hint: "Color (), [] and {} by nesting depth; unmatched brackets use the error color.",
+        },
+      ],
+    },
   ],
 });