Add item filter to donations index page

[costajohnt]

Summary

• Adds by_item_id and by_item_category_id scopes to the Donation model, mirroring the existing scopes on Distribution
• Wires up a "Filter by Item" dropdown on the donations index page using the existing filter_select helper
• Permits the new filter params in both DonationsController#filter_params and View::Donations.filter_params
• Adds model specs for both new scopes and a system spec for the item filter

Closes #5105

Approach

Followed the pattern established by the Distribution model's by_item_id scope (line 54 of distribution.rb) and the Distribution index page's item filter dropdown. The Filterable concern's class_filter method handles scope chaining automatically.

Files changed

File	Change
app/models/donation.rb	Add by_item_id and by_item_category_id scopes
app/models/view/donations.rb	Add items to Data.define, permit new filter params, add selected_item accessor
app/controllers/donations_controller.rb	Permit :by_item_id and :by_item_category_id in filter_params
app/views/donations/index.html.erb	Add "Filter by Item" dropdown
spec/models/donation_spec.rb	Scope specs for by_item_id and by_item_category_id
spec/system/donation_system_spec.rb	System spec for item filtering on donations page

Test plan

• Model spec: by_item_id returns only donations containing the specified item
• Model spec: by_item_category_id returns only donations with items in the specified category
• System spec: selecting an item from the filter dropdown and clicking Filter shows only matching donations
• Existing filter tests continue to pass (category, source, manufacturer, etc.)
• CSV export respects the item filter (handled automatically by class_filter)