Sync highlighted subtitles and add optional end fade

[nickbeau]

Summary

• compensate exported and review subtitle timings by 1.5 seconds for the local speech model's observed recognition delay
• keep the final subtitle line visible for 1.5 seconds after the last timed word
• add a persisted one-second fade-to-black option for both clean and subtitled MP4 exports
• generalize the local render endpoint so clean faded exports and highlighted subtitle exports share the validated FFmpeg path
• document the updated timing and export behavior

Validation

• npm.cmd audit --audit-level=high: passed, 0 vulnerabilities
• npm.cmd test: passed, 57 tests
• npm.cmd run typecheck: passed
• npm.cmd run build:desktop: passed
• git diff --check: passed

Notes

• npm.cmd run speech:smoke could not run on this ARM64 workstation because the active ARM64 Node 26 runtime cannot load the repository's intentionally x64 sherpa native addon. The failure occurs while loading sherpa-onnx-node, before exercising changed code; CI runs this check on x64 Windows.
• Existing untracked Microsoft Store artwork was left untouched and is not included in this PR.