fix(unified-inbox): don't discard every account's envelopes if one account fails

[ktogias]

Summary

fetchEnvelopes() and fetchNextEnvelopes() combine every constituent account's envelope fetch into the unified/priority inbox via Promise.all(), which rejects as soon as any single promise rejects. A single account that's temporarily unreachable (throttled provider, network hiccup, provisioned account with an unavailable server, etc.) was therefore enough to make the whole unified mailbox render nothing at all, discarding every other account's already-successful envelopes too.

This matches the failure shape described in #9072 ("Other section of unified inbox doesn't load for new accounts") — one account's IMAP server being unavailable breaking the combined view for every account.

Fix

Catches each individual account's fetch before it reaches Promise.all(), so a failing account contributes an empty list instead of failing the whole batch. Same reasoning applied to the pagination path in fetchNextEnvelopes().

Test plan

• Added a unit test with two accounts where one throws — confirms the working account's envelopes still render.
• Verified the new test fails without the fix (reproduces the original bug) and passes with it.
• npx vitest --run src/tests/unit/store/actions.spec.js — 24/24 passing.
• npx eslint src/store/mainStore/actions.js — 0 errors (pre-existing unrelated jsdoc warnings only).

Fixes #9072

[ktogias]

You're right, and I'm sorry. I missed the guideline about the disclosure.