feat(main): draggable divider between the two panes

The Main tab was a fixed 50/50 grid, but a map and a cluster list want
very different widths and which one deserves the room changes with what
the operator is doing.

The share is clamped to 15..85: a pane can be made small but never
dragged out of existence, because recovering from that would mean
grabbing a divider no longer on screen. Double-click restores even.

The drag uses pointer capture on the divider — without it Leaflet
swallows the moves as soon as the cursor crosses the map. Persisted
through writeUiPref, so it travels with data/ like the other UI prefs.

frontend/src/lib/i18n.tsx

@@ -51,7 +51,7 @@ const en: Dict = {
   'dup.colDate': 'Date / time (UTC)', 'dup.colFreq': 'Freq', 'dup.colRst': 'RST s/r', 'dup.colName': 'Name',
   'profileScope.saved': 'Saved for profile', 'profileScope.switch': '— switch profiles to edit another identity.',
   // Tabs
-  'tab.main': 'Main', 'tab.recent': 'Recent QSOs', 'tab.cluster': 'Cluster', 'tab.worked': 'Worked before',
+  'tab.main': 'Main', 'main.splitTip': 'Drag to resize the panes — double-click to even them out', 'tab.recent': 'Recent QSOs', 'tab.cluster': 'Cluster', 'tab.worked': 'Worked before',
   'tab.awards': 'Awards', 'tab.bandmap': 'Band Map', 'tab.contest': 'Contest', 'tab.net': 'Net',
   // Entry form
   'field.callsign': 'Callsign', 'field.name': 'Name', 'field.qth': 'QTH', 'field.grid': 'Grid',
@@ -464,7 +464,7 @@ const fr: Dict = {
   'dup.selectExtras': 'Tout sélectionner sauf le premier', 'dup.deselectAll': 'Tout désélectionner', 'dup.deleteSel': 'Supprimer {n} sélectionné(s)', 'dup.deleteConfirm': 'Supprimer {n} QSO ? Action irréversible.', 'dup.deleted': '{n} doublon(s) supprimé(s)', 'dup.keep': 'le plus ancien', 'dup.close': 'Fermer',
   'dup.colDate': 'Date / heure (UTC)', 'dup.colFreq': 'Fréq', 'dup.colRst': 'RST e/r', 'dup.colName': 'Nom',
   'profileScope.saved': 'Enregistré pour le profil', 'profileScope.switch': "— change de profil pour éditer une autre identité.",
-  'tab.main': 'Principal', 'tab.recent': 'QSO récents', 'tab.cluster': 'Cluster', 'tab.worked': 'Déjà contacté',
+  'tab.main': 'Principal', 'main.splitTip': 'Faites glisser pour redimensionner les volets — double-clic pour les égaliser', 'tab.recent': 'QSO récents', 'tab.cluster': 'Cluster', 'tab.worked': 'Déjà contacté',
   'tab.awards': 'Diplômes', 'tab.bandmap': 'Carte de bande', 'tab.contest': 'Contest', 'tab.net': 'NET',
   'field.callsign': 'Indicatif', 'field.name': 'Nom', 'field.qth': 'QTH', 'field.grid': 'Locator',
   'field.band': 'Bande', 'field.mode': 'Mode', 'field.country': 'Pays', 'field.comment': 'Commentaire',