[Clients]: Field-by-field conflict picker for client merge

[nielsdrost7]

RelationMergeService::merge() (Modules/Clients/Services/RelationMergeService.php, introduced in PR #580) always keeps the primary record's value when both records have a conflicting non-empty value for trading_name, unique_name, id_number, coc_number, vat_number, currency_code, language, primary_contact_id — it only gap-fills empty primary fields from the duplicate. The original issue (#481) sketched a UI letting the user choose which side wins per field; that UI was descoped from the initial implementation.

Acceptance criteria

• The merge bulk action (Modules/Clients/Filament/Company/Resources/Relations/Tables/RelationsTable.php) shows a per-field radio/picker for any field where primary and duplicate disagree (both non-empty), before calling RelationMergeService::merge() with the resolved values.
• Existing gap-fill-only behavior remains the default when values don't conflict.