fix: #3615603 Make the allotment screens usable: form fields, add actions and ISO 8601 release offsets

The three entity types added in #3615101 had no widget in yoyaku_ui's map, so their default form display had no components and ContentEntityForm rendered nothing but Save. No allotment could be created at all, and no collection offered an add action.

• Widgets for yoyaku_allotment, yoyaku_resource_allotment and yoyaku_slot_allotment.
• The allotment reference is a select rather than an autocomplete, for the reason the tariff class is one: a tenant names a handful of allotments and they are its own vocabulary. Applied to the preset allocation form too, so the field looks the same wherever it is named.
• Add actions for the six collections that lacked one (the three allotment types, preset allocations, preset tariffs, slot tariffs).
• A release offset is typed as seconds or as an ISO 8601 duration such as P1D, read by a shared Duration::toSeconds() that the settlement grace now uses too. The column keeps holding seconds, so the availability query goes on doing integer arithmetic on it.
• The pinnings table called its count "Places held", the label an allotment's capacity field carries. It becomes "Pinned places".

Docs and fr.po updated in the same commit. AllotmentAdminFormTest was seen to fail against the unfixed code on the three assertions that matter, and passes with the fix.