feat(sync): the folder loop, the hooks and the panel — it can be switched on now

The three layers underneath were already there and unreachable: the change-log
format (7d664bd), the identity column (724e68b) and the no-backfill decision
(c48294b). This is the wiring that gives the operator a switch.

Settings, all profile-scoped, because each profile can point at its own logbook:
the folder, this PC's name, the machine id minted once from it, the per-peer read
offsets and the tie-break counter. Scoping the machine id is what keeps two
profiles sharing one folder from writing two logbooks into one file.

Three hooks. Add and update publish asynchronously, down with the rest of the
after-the-fact work — a folder on a network share can block for seconds and a
contact belongs on screen long before another machine hears about it. Deletion
publishes SYNCHRONOUSLY and BEFORE the row goes, for the same reason
deleteRemoteCopies does: once it is gone its identity is gone with it and the
tombstone names nothing.

The apply path uses the repository directly and never AddQSO/UpdateQSO/DeleteQSO
— those publish, and a change applied here would be written straight back out,
two machines echoing each other for ever.

Saving writes a probe file to the chosen folder rather than asking whether it
exists. A read-only cloud folder, or a share whose credentials expired, exists
perfectly well and would swallow every contact in silence; if the probe fails the
switch goes back off instead of sitting on while nothing is written.

The panel is mostly status, and deliberately: every part of this runs on another
machine and on a sync client OpsLog cannot see, so "it is not working" has to be
answerable from the settings page — which PCs are in the folder, when each last
logged, what is waiting unread.

Four tests on the apply path, the middle two being the ones that matter: an edit
made on the other PC lands on the copy already here, matched on the contact
itself, instead of becoming a second row — that is what makes the no-backfill
decision safe — and a contact with the same station on another band stays a
separate contact.

frontend/src/lib/i18n.tsx

@@ -118,6 +118,7 @@ const en: Dict = {
   'sec.bands': 'Bands', 'sec.modes': 'Modes & default RST', 'sec.cluster': 'DX Cluster',
   'sec.udp': 'Connections', 'sec.database': 'Database', 'sec.autostart': 'Autostart', 'sec.backup': 'Database backup', 'sec.uscounties': 'US Counties',
   'sec.webpublish': 'Web publishing', 'wpub.hint': 'Publishes your log as a file for a website: a standalone HTML page or a CSV, written locally and optionally uploaded by FTP. It is refreshed when you log a QSO and, if you set an interval, on a timer.', 'wpub.enable': 'Publish the log to a file', 'wpub.fileSection': 'The file', 'wpub.format': 'Format', 'wpub.formatHtml': 'HTML page', 'wpub.formatCsv': 'CSV', 'wpub.folder': 'Output folder', 'wpub.browse': 'Browse…', 'wpub.fileName': 'File name', 'wpub.title': 'Page title', 'wpub.titlePh': 'blank = your callsign', 'wpub.count': 'Last N QSOs', 'wpub.every': 'Refresh every', 'wpub.everyHint': 'minutes — 0 = only when a QSO is logged', 'wpub.columns': 'Columns', 'wpub.columnsCount': '{n} of {total} chosen', 'wpub.columnsPick': 'Choose columns…', 'wpub.columnsSearch': 'Search a field…', 'wpub.removeColumn': 'Click to remove', 'wpub.columnsHint': 'Click to add or remove. The order shown here is the order in the file.', 'wpub.ftpEnable': 'Upload by FTP', 'wpub.ftpHost': 'Server / port', 'wpub.ftpUser': 'User', 'wpub.ftpPassword': 'Password', 'wpub.ftpFolder': 'Remote folder', 'wpub.ftpFileName': 'Remote file name', 'wpub.ftpTls': 'Use TLS (FTPS)', 'wpub.publishNow': 'Publish now', 'wpub.testFtp': 'Test connection', 'wpub.lastRun': 'Last run:', 'sec.adifmon': 'ADIF monitor',
+  'sec.foldersync': 'Sync across PCs', 'sync.hint': 'Point every OpsLog at the SAME folder — one your PCs already synchronise (Seafile, OneDrive, Dropbox, a NAS share). Each machine writes what it logs there and reads the others; the databases themselves are never shared.', 'sync.enable': 'Keep my contacts in step across my PCs', 'sync.machine': 'This PC', 'sync.folder': 'Folder', 'sync.choose': 'Choose…', 'sync.state': 'State', 'sync.thisPc': 'This PC', 'sync.lastSync': 'Last check', 'sync.sent': 'Sent', 'sync.received': 'Received', 'sync.never': 'never', 'sync.noPeers': 'No other PC has written to this folder yet.', 'sync.behind': 'new contacts waiting', 'sync.now': 'Synchronise now', 'sync.applied': '{n} change(s) taken from the folder.', 'sync.saved': 'Saved.',
   'adifmon.hint': 'Watch external ADIF files and import new QSOs automatically — e.g. fldigi logging RTTY, or N1MM/VarAC. Imported QSOs are enriched, de-duplicated and uploaded to your external services just like a QSO logged here.',
   'adifmon.enable': 'Enable ADIF monitor',
   'adifmon.empty': 'No file watched yet. Add an ADIF file below.',
@@ -555,6 +556,7 @@ const fr: Dict = {
   'sec.bands': 'Bandes', 'sec.modes': 'Modes & RST par défaut', 'sec.cluster': 'DX Cluster',
   'sec.udp': 'Connexions', 'sec.database': 'Base de données', 'sec.autostart': 'Démarrage auto', 'sec.backup': 'Sauvegarde base', 'sec.uscounties': 'Comtés US',
   'sec.webpublish': 'Publication web', 'wpub.hint': "Publie ton journal dans un fichier destiné à un site web : une page HTML autonome ou un CSV, écrit en local et envoyé par FTP si tu le souhaites. Il est rafraîchi à chaque QSO enregistré et, si tu règles un intervalle, périodiquement.", 'wpub.enable': 'Publier le journal dans un fichier', 'wpub.fileSection': 'Le fichier', 'wpub.format': 'Format', 'wpub.formatHtml': 'Page HTML', 'wpub.formatCsv': 'CSV', 'wpub.folder': 'Dossier de sortie', 'wpub.browse': 'Parcourir…', 'wpub.fileName': 'Nom du fichier', 'wpub.title': 'Titre de la page', 'wpub.titlePh': 'vide = ton indicatif', 'wpub.count': 'N derniers QSO', 'wpub.every': 'Rafraîchir toutes les', 'wpub.everyHint': 'minutes — 0 = seulement à chaque QSO', 'wpub.columns': 'Colonnes', 'wpub.columnsCount': '{n} sur {total} choisis', 'wpub.columnsPick': 'Choisir les colonnes…', 'wpub.columnsSearch': 'Chercher un champ…', 'wpub.removeColumn': 'Cliquer pour retirer', 'wpub.columnsHint': 'Clique pour ajouter ou retirer. L ordre affiché ici est celui du fichier.', 'wpub.ftpEnable': 'Envoyer par FTP', 'wpub.ftpHost': 'Serveur / port', 'wpub.ftpUser': 'Utilisateur', 'wpub.ftpPassword': 'Mot de passe', 'wpub.ftpFolder': 'Dossier distant', 'wpub.ftpFileName': 'Nom du fichier distant', 'wpub.ftpTls': 'Utiliser TLS (FTPS)', 'wpub.publishNow': 'Publier maintenant', 'wpub.testFtp': 'Tester la connexion', 'wpub.lastRun': 'Dernière exécution :', 'sec.adifmon': 'Moniteur ADIF',
+  'sec.foldersync': 'Synchro entre PC', 'sync.hint': 'Fais pointer chaque OpsLog vers le MÊME dossier — un dossier que tes PC synchronisent déjà (Seafile, OneDrive, Dropbox, un partage NAS). Chaque machine y écrit ce qu’elle enregistre et lit celui des autres ; les bases de données, elles, ne sont jamais partagées.', 'sync.enable': 'Garder mes contacts à jour sur tous mes PC', 'sync.machine': 'Ce PC', 'sync.folder': 'Dossier', 'sync.choose': 'Choisir…', 'sync.state': 'État', 'sync.thisPc': 'Ce PC', 'sync.lastSync': 'Dernière vérification', 'sync.sent': 'Envoyés', 'sync.received': 'Reçus', 'sync.never': 'jamais', 'sync.noPeers': 'Aucun autre PC n’a encore écrit dans ce dossier.', 'sync.behind': 'nouveaux contacts en attente', 'sync.now': 'Synchroniser maintenant', 'sync.applied': '{n} changement(s) repris du dossier.', 'sync.saved': 'Enregistré.',
   'adifmon.hint': "Surveille des fichiers ADIF externes et importe les nouveaux QSO automatiquement — ex. fldigi en RTTY, ou N1MM/VarAC. Les QSO importés sont enrichis, dédoublonnés et envoyés à tes services externes comme un QSO loggé ici.",
   'adifmon.enable': 'Activer le moniteur ADIF',
   'adifmon.empty': 'Aucun fichier surveillé. Ajoute un fichier ADIF ci-dessous.',