Add notification sound volume and custom sounds

[ChocoTonic]

Add notification sound volume and custom sounds

Summary

• Add a 0–100 new-mail notification volume slider in General > Notifications.
• Add a native custom-sound picker for MP3, OGG, WAV, M4A, AAC, and FLAC files,
  with Play Test Sound and reset-to-default actions.
• Keep both settings scoped to new-mail notifications so sent-message and other UI
  sounds retain their existing behavior.
• Resolve custom files at playback time and fall back to the bundled sound when a
  selection is missing, unsupported, or cannot be played.
• Permit local files only as media resources, without expanding the script policy,
  and reject relative paths, non-regular files, empty files, and files over 25 MiB.
• Extend SoundRegistry with reusable per-play volume and source options, keeping
  the preference UI, sound selection, and audio playback concerns separate.
• Add regression coverage for the silent Windows toast behavior established by
  Silence audio in native Windows notifications #2619, which ensures native toast audio does not bypass Mailspring's playback
  settings.

The work is organized as two focused commits in this PR: the first introduces the
volume control and reusable playback option, and the second adds custom sound
selection on top of that foundation.

Testing

• Added registry coverage for muted, intermediate, full, invalid, and out-of-range
  volume values, custom sources, and decode-error fallback.
• Added resolver coverage for supported extensions, file URLs, missing files, and
  unsupported selections.
• Added preference UI coverage for slider persistence and choose, reset, and test
  actions.
• Added new-mail integration coverage showing that configured sound options apply
  only to notification playback.
• Added regression coverage for silent individual and summary Windows toast XML.
• npm run lint:check
• Targeted Electron specs: 31 passing
• Real external-file playback in Electron: MP3, OGG, WAV, M4A, AAC, and FLAC all
  passed through the production resolver and Content Security Policy.
• npm run build (produced the macOS arm64 app and ZIP)

npm run typecheck reaches the full project and reports three existing errors
outside this change: the marginType option in print-window.ts and two inert
attribute errors in sheet-container.tsx. It reports no errors in files changed by
this PR.

Feature requests and related material

• Volume support request: https://community.getmailspring.com/t/how-do-i-reduce-the-volume-of-the-sound-when-receiving-new-mail/1639/2
• Custom notification sound discussion: https://community.getmailspring.com/t/custom-notification-sound/530
• Existing bundled-file replacement workaround: https://community.getmailspring.com/t/custom-notification-sound/530/2
• Windows/system-sound discussion: https://community.getmailspring.com/t/custom-notification-sound/530/5
• Original GitHub feature request: [Feature Request] Set custom notification sound for new emails #415
• Related Windows sound-routing prior art: Silence audio in native Windows notifications #2619

Per-filter notification sounds remain out of scope for this change.

[ChocoTonic]

Implemented the notification sound settings follow-up and tested it in a packaged Windows build.

• Grouped notification sound selection, reset, volume, and preview controls.
• Kept Choose Sound… and Use Default Sound together as setting-changing actions.
• Moved Play Sound to a separate row because it previews the current selection.
• Made Choose Sound… the emphasized action.
• Displayed the current filename as the setting value; the reset state displays Default notification sound without exposing the bundled filename.
• Made Play Sound and actual new-mail notifications use the selected custom sound, falling back to the bundled default only when needed.
• Allowed selected local audio files through the app's media CSP.
• Opened the file picker at the selected sound or the bundled default sound location using a relative runtime path.

Related community request: https://community.getmailspring.com/t/add-a-receive-sound-that-is-selectable-for-incoming-email/14455

[CEOYap]

Looking forward to this!