Import contacts from CSV or JSON file

Bulk import from arbitrary CSV or JSON. An AI pass reads the first 10 lines / 500 chars and auto-maps unknown column names to canonical contact fields (firstName, lastName, phoneNumber, email, address, companyName, notes, tags). If mapping fails, sensible defaults are used. Rows missing a valid phone, or missing BOTH firstName and companyName, are rejected with per-row errors; valid rows merge by phone rather than creating duplicates. Comma-separated tag strings are split into arrays.