feat(logging): persist per-QSO transmit power (tx_pwr) end to end (#244)

The "Power (W)" field on the log form was collected and POSTed as `power`,
but no column consumed it and `Contact.create` never inserted it — so an
operator's per-QSO power was silently dropped on save, and ADIF export could
only emit the station's single default power.

- schema: add nullable `contacts.tx_pwr numeric(10,2)` (ADIF TX_PWR, watts) +
  migration 0006_add_contacts_tx_pwr.
- Contact.create: persist tx_pwr.
- new-contact form: send the ADIF-standard `tx_pwr` at the wire (component
  state stays `power`, which is form-local and exempt from snake_case).
- ADIF import: read TX_PWR so imported logs keep per-QSO power.
- ADIF export: COALESCE(c.tx_pwr, s.power_watts) so a per-QSO value wins and
  falls back to the station default; the search export's SELECT * carries it
  automatically.
- EditContactDialog: expose Power (W) so a logged/imported value is visible
  and editable (blank clears to NULL, not an empty string).
- tests: guard tx_pwr export + parser round-trip in adif-generate.spec.

Co-authored-by: Optio Agent <optio-agent@noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

drizzle/migrations/0006_add_contacts_tx_pwr.sql

@@ -0,0 +1 @@
+ALTER TABLE "contacts" ADD COLUMN "tx_pwr" numeric(10, 2);