feat: Limit transactions to one category and add tags

[nfebe]

Limit transaction entry to a single category and add tags, matching the new server model, and drop the on-device aggregation that adds a transaction's amount to every category it carries. This is the category half of the model change; the transfer, month, currency, and sync gaps are handled separately in #209.

Blocked by trakli/webservice#295.

Checklist

• Category selection allows one category per transaction on create and edit.
• Add tags as a separate label: local Drift table plus sync, independent of categories.
• Attribute each transaction to its one category in Reports and Month-in-Review; remove the add-to-every-category loop (report_data.dart:138-140, month_in_review_data.dart:112-114).
• Migrate the local schema and existing multi-category links to the one-category rule.

Acceptance criteria

• A transaction carries one category and any number of tags, locally and after sync.
• Category figures match the server once sync has settled.