feat(ui): keyboard overhaul — shared instance, icon keys, symbols page

- Unify on the shared keyboard: WaypointsView drives UITask::keyboard()
  instead of its own instance; openKb() clears {loc}/{time} for literal
  fields (labels, coordinates). Reclaims the duplicate widget from the heap.
- Icon glyphs on the special row: caps ⇧, space ⎵ (two halves), delete ⌫,
  OK ✓ (reuses ICON_CHECK); {} and the page toggle stay text.
- Second character page (symbols), toggled by a "#@"/"abc" key. Tidy the
  letters page: drop the duplicate grid space, add the comma, group
  punctuation as . , ! ?
- Harden the preview buffers (KB_PREVIEW_CAP) against very wide displays.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.h

@@ -154,6 +154,8 @@ public:
   void gotoCompassScreen();
   TrailStore& trail() { return _trail; }
   WaypointStore& waypoints() { return _waypoints; }
+  // Shared on-screen keyboard — only one screen drives it at a time.
+  KeyboardWidget& keyboard() { return _kb; }
   void saveWaypoints();
   // Add a waypoint, persist, and show the standard "Waypoint saved" / "Waypoints
   // full" alert. Returns true on success. The ts-less overload uses current RTC time.