feat(web-test): add showImage/hideImage for displaying images during recording

Show image files (PNG, JPG, etc.) as full-screen overlays during video
recording — useful for presentation slides in video instructions.

- Read file → base64 → inject as <img> overlay (same pattern as showTitleSlide)
- Style presets: blur (default), dark, light, full
- blur: blurred+dimmed copy as background with shadow
- full: object-fit cover, fills entire screen
- TTS speech support with smart wait (same as showCaption)
- Custom background overrides preset
- Fixed no-record stubs: showImage/showTitleSlide not stubbed (visual-only)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude/skills/web-test/scripts/run.mjs

@@ -124,7 +124,7 @@ async function executeScript(code, { noRecord } = {}) {
       exports.startRecording = noop;
       exports.stopRecording = async () => ({ file: null, duration: 0, size: 0 });
       exports.addNarration = async () => ({ file: null, duration: 0, size: 0, captions: 0 });
-      for (const fn of ['showCaption', 'hideCaption', 'showTitleSlide', 'hideTitleSlide']) {
+      for (const fn of ['showCaption', 'hideCaption']) {
         exports[fn] = noop;
       }
       exports.isRecording = () => false;