[paultristanwagner]

All code in this PR was written by an AI agent (Claude Opus 5.5) working with me; I directed it and tested the results on my own account. Feel free to take, change or drop any part of it.

Part of #73.

Adds an update_food_entry tool: change a logged entry's amount or
measure, or move it to another meal. Until now the only way was to remove
the entry and log it again.

The app changes an amount with POST /api/v2/edit_serving, sending the whole serving back with the
new grams:

{"serving": {"servingId": …, "day": …, "order": 65537, "foodId": …,
             "measureId": …, "grams": 400.0, …},
 "auth": {…}, "config": {"call_version": 2}}

The meal group lives in the high bits of order, as add_serving already
builds it ((group << 16) | position). The app moves an entry with a
separate POST /api/v2/update_order {"diaryEntries": [<serving with new order>]}, but edit_serving accepts a new order too, so one call does
both. Verified against the live API on 1900-01-02: amount, meal and both at
once.

Changes:

• client: update_serving(serving_id, day, *, grams, measure_id, diary_group) reads the serving from get_diary, changes the given
  fields and sends it back. A moved entry goes to the end of its new meal;
  staying in the same meal keeps its position. Arguments are validated
  before any request.
• server: update_food_entry tool. The meal-name map that
  add_food_entry built inline is now a module constant both tools use.
• README: tool table row.

https://claude.ai/code/session_01MCJFDo71qoSqTRtcGfkpXJ