Fix/GitHub returning null

[AmarBego]

What changed?

1. GitHub can return repository.owner: null in a successful notifications response. Our model required owner to always be
   present, so deserialization failed and the whole refresh errored. The fix makes owner optional and uses an empty
   avatar fallback.
2. This also adds optional repository-owner avatars in the notification detail panel. Avatars are enabled by default, can be disabled in General settings, and are only loaded for the currently selected notification
3. Added graceful 404 handling for deleted or missing notification targets: the detail panel now shows a clear “Details unavailable” state with the GitHub error code instead of falling back to an empty/generic view.
4. Low-memory/tray mode now drops transient detail state, including avatar handles, loaded comments, labels, checks, selection state, and pending UI state.

Why?

1. A single malformed or partial notification response should not prevent the entire inbox from refreshing.
2. The avatar handling needed to be defensive too: if GitHub omits owner data, avatar loading is disabled, or an avatar fetch fails, the UI falls back to initials instead of retaining image bytes or blocking the detail view.
3. more of a quality of life change, tells you the notification is broken (deleted repo, deleted issue etc.)

How was this tested?

fully

Platforms tested

• Windows
• Linux
• FreeBSD

Fixes (if applicable)

Fixes #