The notice promised 'this will clear once it's back', but nothing ever
removed it: the toast auto-hid on a 15s timer (not on recovery) and the
v0.9.0 unified inbox kept every notify() in persistent history, so the
notice lingered there forever.
Model it as a single live status instead: one sticky, transient (never
persisted) notice while the server is unreachable, removed the moment any
request reaches the server again. Adds a 'transient' notification flag
(sticky toast + excluded from history so a reload can't orphan it) and
replaces the throttled-toast helper with a connectivity lost/restored pair.
i18n the strings (errors.offline.*) in EN/HU/DE — they were hardcoded English.
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