feat(ui): Limit transactions to one category and add tags

[nfebe]

Limit transaction entry to a single category and surface tags for everything else, matching the new server model, and drop the client-side handling that collapses multiple categories. Reports read the single-category breakdown from /stats rather than deriving it locally.

Blocked by trakli/webservice#295.

Checklist

• Category picker allows one category per transaction on create and edit.
• Add a multi-tag input on transactions, backed by the tags API.
• Reports and breakdowns read the single-category /stats output; remove local multi-category logic in useReportData.ts.
• Show tags where they help: transaction list, detail, and filters.

Acceptance criteria

• A transaction has at most one category and any number of tags.
• Category breakdowns match /stats.