fix: tighten Electron API typings for typecheck

[RutamBhagat]

Summary

• tighten Electron bridge typings in src/types/electron.ts to match actual return contracts
• add ModelDownloadResult and use it for modelDownload instead of Promise<void>
• remove unnecessary cast in useModelDownload now that the API is correctly typed

Stacked PR Context

This PR depends on fix/typecheck-1 and should be reviewed after that PR.

Depends on #209

Output

npm run typecheck

Found 18 errors in 8 files.

Errors  Files
     1  src/components/LocalWhisperPicker.tsx:168
     3  src/components/SettingsPage.tsx:1447
     4  src/components/TitleBar.tsx:41
     5  src/components/TranscriptionModelPicker.tsx:47
     1  src/components/ui/MarkdownRenderer.tsx:12
     1  src/helpers/llamaCppInstaller.ts:8
     1  src/lib/neonAuth.ts:119
     2  src/services/ReasoningService.ts:17