Say what an empty Places held does, and how it differs from 0

Problem/Motivation

On a resource allotment, the Places held field is described as "How many places every session of this resource holds under the allotment. Leave empty to declare no number here." The second sentence restates that the field is empty and never says what follows from it, so nothing on the form tells an operator apart from an empty value and a 0.

The two are not the same, and the difference is load-bearing wherever an allotment owns named places:

• empty withholds no number, which lets the places pinned to the allotment be all it holds
• 0 holds nothing at all, pinned places included

The preset row carries the same wording, and the session record never mentions 0 even though 0 is how one evening says it holds nothing tonight.

Proposed resolution

Reword the three Places held descriptions so each says what its own empty value does, and what 0 does beside it. The counting is unchanged; the French translation moves with the strings in the same commit.

Remaining tasks

Review.

User interface changes

The Places held field descriptions on a resource allotment, a session allotment and a tariff preset row.

API changes

None.

Data model changes

None.

Release notes snippet

The Places held field now says what leaving it empty does, and how that differs from holding 0.

AI-Generated: Yes (Claude Code was used to read the counting code, to draft this issue summary, and to write the reworded descriptions and their French translations. I reviewed all of it. The change is field-description text only, so it carries no test.)