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/uiPref.ts

@@ -36,6 +36,7 @@ const PORTABLE_KEYS = [
   'opslog.clusterLockBand', 'opslog.clusterLockMode', 'opslog.clusterStatusFilter',
   'opslog.clusterModeFilter', 'opslog.clusterSearch', 'opslog.clusterHideWorked',
   'opslog.activeTab',     // last selected tab
+  'opslog.mainSplit',    // Main tab: width share of the left pane (percent)
   // NOTE: 'hamlog.awardColsShown' and the grid column layouts are NOT listed here.
   // They are handled by lib/gridPrefs, which scopes the localStorage cache PER
   // PROFILE and mirrors to the DB (already per-profile) itself — mirroring them