feat(dives): a dive whose time of day is unknown shows and edits as a date

[aleskiontherun]

The API now serves a bare YYYY-MM-DD start_time for an imported dive whose time of day was never recorded, and keeps that state for as long as PATCH /dive/{uuid} receives the bare date back. isDateOnlyStartTime names the shape; formatDiveDateTime drops the clock for it and formatDiveStartTime stops after the date, so the list, the page header, the neighbour and merge labels, the renumber preview and the gas-use chart show a date instead of a midnight nobody recorded. splitStartTime returns the date without a time, so the edit form echoes it unchanged, and DiveStartTimeField shows the date, an empty time box and a disabled offset select; typing a time sends an offsetless date-time, which ends the state. A file that states only a day, applied to such a dive, keeps it a date. The create form is unchanged.

Verified with npm run lint, type-check, test (the date tests also under TZ UTC, America/Los_Angeles and Pacific/Kiritimati) and build.

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