Revert "feat: remove PostHog usage telemetry entirely"

This reverts commit 6a28344. Restore the once-a-day anonymous PostHog heartbeat
and its opt-out toggle: telemetry.go (sendTelemetryHeartbeat, GetTelemetryEnabled/
SetTelemetryEnabled, PostHog host + API key), the startup goroutine, the Settings
→ General toggle + i18n keys, and the README/wiki mentions. version.go is dropped
again (appVersion moves back into telemetry.go). release.ps1 (untracked) pointed
back at telemetry.go by hand.

frontend/src/lib/i18n.tsx

@@ -98,6 +98,8 @@ const en: Dict = {
   'offline.synced': '{n} QSO(s) added to the logbook',
   'offline.stillDown': 'Database still unreachable — your QSOs are safe',
   'settings.theme': 'Theme', 'settings.themeHint': 'Interface colour theme.',
+  'settings.telemetry': 'Send anonymous usage statistics',
+  'settings.telemetryHint': 'install ID + version + OS, once a day — no callsign or QSO data',
   'settings.mainView': 'Main view',
   'settings.mainViewHint': 'Choose what the Main tab shows on each side (per profile).',
   'settings.leftPane': 'Left pane', 'settings.rightPane': 'Right pane',
@@ -465,6 +467,8 @@ const fr: Dict = {
   'offline.synced': '{n} QSO ajoutés au journal',
   'offline.stillDown': 'Base toujours injoignable — tes QSO sont en sécurité',
   'settings.theme': 'Thème', 'settings.themeHint': "Thème de couleur de l'interface.",
+  'settings.telemetry': "Envoyer des statistiques d'usage anonymes",
+  'settings.telemetryHint': "ID d'installation + version + OS, une fois par jour — aucun indicatif ni donnée QSO",
   'settings.mainView': 'Vue principale',
   'settings.mainViewHint': "Choisis ce que l'onglet Principal affiche de chaque côté (par profil).",
   'settings.leftPane': 'Volet gauche', 'settings.rightPane': 'Volet droit',