Optimization: Move ADIF import off the UI thread

[ea4k]

Problem

The ADIF import loop in src/filemanager.cpp:872-933 parses the file and inserts every QSO synchronously on the main (GUI) thread. The work is correctly wrapped in a single transaction (beginTransaction() at src/filemanager.cpp:870), but the event loop is still starved for the whole duration.

filemanager.cpp contains no QThread/QtConcurrent usage, even though the codebase already uses QtConcurrent::run elsewhere (e.g., src/dataproxy_sqlite.cpp:69, :9029), so the pattern is established in the project.

User-visible symptom

Importing a 5k–50k QSO ADIF file freezes the window for tens of seconds to minutes. The app appears hung ("not responding"), and users may force-kill it mid-import.

Proposed fix

• Run the parse + insert loop in a worker via QtConcurrent::run (note: SQLite/QSqlDatabase requires a separate connection per thread).
• Report progress back through queued signals to the existing progress dialog (the progress UI infrastructure is already there).
• Refresh log model / awards / DXCC widgets once at the end, not per QSO.

Combined with the missing-index fix on the log table (duplicate checks during import are currently full-table scans), import time should drop dramatically and the UI stays responsive throughout.