Fix macOS clipboard restore delay

[SyntaxSawdust]

Summary

• Restore the macOS clipboard after 450ms instead of 2000ms after a successful paste.
• Add coverage for both the fast-paste path and the osascript fallback to keep the shared macOS restore delay at 450ms.

Why

Fixes #1023.

OpenWhispr 1.7.3 increased the macOS clipboard restore delay to 2000ms. That leaves the dictated text on the clipboard for longer after paste, which can cause follow-up paste behavior to use the dictated text rather than the user's previous clipboard content. This restores the shorter macOS delay while keeping the existing expected-text guard that avoids overwriting a newer user clipboard write.

Tests

• mise x node@24 -- node --test test/helpers/clipboardRestore.test.js passed, 5/5.
• mise x node@24 -- npm run lint passed.
• mise x node@24 -- npm run typecheck passed.
• mise x node@24 -- npx prettier --check src/helpers/clipboard.js test/helpers/clipboardRestore.test.js passed.
• git diff --check upstream/main passed.
• mise x node@24 -- npm run format:check ran but failed on existing unrelated formatting issues in helpers/audioManager.js, helpers/speakerEmbeddings.js, and helpers/textEditMonitor.js; the two changed files pass Prettier directly.

Scope / non-goals

• Scope is limited to the macOS clipboard restore delay and focused unit coverage for the macOS paste success paths.
• No changes to Windows or Linux clipboard timing.
• No changes to paste behavior beyond the restore delay value.

Caveats

• I did not run a manual macOS dictation smoke test.
• PR fix: preserve rich clipboard formats on restore #1020 also touches src/helpers/clipboard.js and test/helpers/clipboardRestore.test.js for Clipboard restore step destroys the formatted-data of the original clipboard contents #834 rich clipboard restoration. This PR fixes a separate [macOS] Dictation pastes twice / previous clipboard not available — 1.7.3 raised the restore delay 450ms → 2000ms #1023 timing regression; if fix: preserve rich clipboard formats on restore #1020 lands first, I will rebase this branch before requesting review.

[gabrielste1n]

thank you for the awesome work here @SyntaxSawdust !!