Upgrade @vuepic/vue-datepicker 11.0.2 → 14.0.0

# Upgrade @vuepic/vue-datepicker 11.0.2 → 14.0.0 (major)

Dependabot PR #7490. Runtime `dependencies` entry (`^11.0.2`); date picker component used only in `frontend/src/ui-components/components/form/DateTime.vue`. Pulled out of the weekly Dependabot slot per the major-version process — needs an owner and verification, not a batch merge.

## Compatibility pre-check

- ✅ Vue: peer `>=3.5.0`, installed `3.5.38`.
- ✅ Node: engines `>=18.12.0`, dev default Node 24.
- ⚠️ Custom SCSS selector `.dp__theme_light` in `ff-vue-datepicker.scss` uses the old underscore class naming — renamed to dash in 13.x (see below). `--dp-*` CSS variables are unaffected.

## Breaking changes to review

- 12.0.0: "a lot of internal refactor and a lot of breaking changes" — to confirm from migration guide.
- 13.0.0: all class names renamed from `underscore` to `dash`; some class wordings changed/removed. `week-start` now derived from the `locale` prop instead of defaulting to Monday.
- 14.0.0: flow API — `handleFlow` renamed to `executeFlow`, takes a step value not index; `partial` removed from the `flow` prop; `@flow-step` emits the step value not index. DateTime.vue does not use flow, so no direct impact.

## To do before merge

- [ ] Visually verify each date picker usage (props/slots/events).
- [ ] Reconcile renamed props/slots.
- [ ] Read the 12.x–14.x migration notes.

## Refs

- Branch: `dependabot/npm_and_yarn/vuepic/vue-datepicker-14.0.0`
- PR: #7490
- Upstream: https://github.com/Vuepic/vue-datepicker/releases

Start with frontend/src/ui-components/components/form/DateTime.vue and ff-vue-datepicker.scss, then read the @vuepic/vue-datepicker 12.x–14.x migration notes. Run the frontend and visually verify each date picker usage, including its props, slots, events, and styling. Done means the upgrade works without incompatibilities and the renamed SCSS selector is reconciled.

Written by the indexing model from the issue text.