test: stub Range layout methods once in the test setup (#53)

Root cause: jsdom has no layout and CodeMirror measures text ranges in an animation frame, sometimes after a test finished; test files that did not stub Range.getClientRects (e.g. search/codePreview) threw an unhandled TypeError that failed whole CI runs at random (seen on #41).
Fix: stub Range.getClientRects / getBoundingClientRect once in src/test/setup.ts; drop the six per-file copies.
Verified: vitest on editor/lsp, editor/refactor, templates, search (55 files, 458 tests, no unhandled errors).

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/features/templates/__tests__/editor.test.ts

@@ -17,11 +17,6 @@ import { closeChooser, useChooser } from "../popup/store";
 
 const MODES: Record<string, string> = { typescript: "TypeScript", rust: "Rust", python: "Python", java: "Java" };
 
-// jsdom has no layout; CodeMirror measures ranges when scrolling the caret into view.
-const rects = () => Object.assign([], { item: () => null }) as unknown as DOMRectList;
-Range.prototype.getClientRects ??= rects;
-Range.prototype.getBoundingClientRect ??= () => new DOMRect();
-
 let view: EditorView | null = null;
 afterEach(() => {
   view?.destroy();