Drives Sessions.tsx and SessionForm.tsx to 100% stmts/branch/funcs/lines.
Added gap tests: sessions error state, in-filter empty message, row
placeholders (missing worker/activity/type/date), frozen worked-time for a
paused active session, edit-form open, and form cancel; SessionForm create/edit
validation paths, non-400 error surfacing, null serialization (empty zooltype +
notitie, status=active clears end_time), pair/pause floors, and the defensive
no-session edit guard.
Lifecycle test (Sessions.test.tsx): open create form -> fill a valid manual
session -> it appears in the list -> edit the row -> the change persists, driven
by a stateful apiFetch mock.
Annotated one unreachable branch: localToIso's Number.isNaN guard is c8-ignored
(a datetime-local input only yields a valid datetime or empty).
No bugs found; no behavior changes.
e7eefe8f62