Stop per-store add forms jumping to the bottom of the page

The inline "Add to <store>..." forms submitted with a bare
hx-swap="innerHTML" against #shopping-items-list. Those forms live *inside*
that target, so the swap destroys the form that submitted it and the page ends
up scrolled to the bottom rather than staying on the store being added to. The
quick-add form at the top never had this because it sits outside the target and
already carried an explicit scroll: modifier.

Each store section now has a stable id and its inline form scrolls back to it,
so adding to a store keeps that store in view.

Adds a `slug` template function for the id: store names are user data
("Trader Joe's", "Hardware Store") going straight into element ids, so anything
outside [a-z0-9-] is collapsed to a dash and an empty result becomes "unnamed"
rather than an invalid, shared anchor.

Tests render the real template with the real func map, and assert every inline
add form has a scroll target — the bug is trivially reintroduced by copying a
section.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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