feat: optional checkbox mode for shopping list (list view)

[sourdusk]

What

Adds an opt-in Checkboxes in list view setting (Settings → Shopping list section, only visible when list view is enabled).

• Each shopping list row gets a leading checkbox. Tap the checkbox → item is removed (same optimistic remove as tap-to-remove mode).
• Tap the row → opens the item page, i.e. exactly what a long-press does today.

Why

Tap-to-remove makes removing fast but hides item details behind a long-press, which is hard to discover. This mode gives both affordances distinct, visible targets: checkbox to check off, row to inspect.

Implementation notes

• New shoppingListCheckboxMode preference (SettingsCubit/SettingsState), default false — no behavior change unless enabled.
• SelectableButtonListTile gains onCheckboxPressed; when set, the leading slot renders an unchecked Checkbox. Confirm-mode's selected check icon still takes precedence.
• Passed through ShoppingItemWidget → SliverItemGridList → SliverCategoryItemGridList → SliverShopinglistItemView; only wired in the shopping list page.
• Row-tap = item page falls back to the existing null-onPressed fallback in SliverItemGridList (the same path a long-press uses), so no duplicated navigation logic.
• Recent items and search results are intentionally unchanged (they're add-flows, not check-off flows).
• Grid view unaffected (tiles have no leading slot).
• l10n: new keys in app_en.arb only ( untranslated-key placeholders: en ).

Testing

• flutter analyze — no new issues (2 pre-existing warnings in untouched files)
• flutter test — all 16 tests pass