feat(ui): full e-ink refresh on screen change + prune gone favourites

Two UI quick wins from the 2026-07-05 review:

- E-ink: force a full (non-partial) refresh on the first frame of a new
  screen. Inter-screen ghosting was the most visible cheap win — the
  N-partials interval alone doesn't catch navigation. New
  DisplayDriver::forceFullRefresh() (no-op on OLED), set in setCurrScreen()
  and consumed by GxEPDDisplay::endFrame().

- Favourites: clear a stale "(gone)" tile at render time so it reverts to an
  empty "+" slot. Happens when prefs outlive the contact list (e.g. a wiped
  /contacts3); onContactRemoved only catches a live delete. Pruned slots are
  persisted once per pass (self-healing — an emptied slot can't re-fire).

Builds green: WioTrackerL1_companion_solo_dual (OLED),
WioTrackerL1Eink_companion_solo_dual (e-ink).

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

src/helpers/ui/DisplayDriver.h

@@ -272,6 +272,7 @@ public:
   virtual void setBrightness(uint8_t level) { }  // level 0-4 (min to max), no-op default
   virtual void setDisplayRotation(uint8_t rot) { }  // 0-3, no-op for fixed-orientation displays
   virtual void setFullRefreshInterval(uint8_t n) { }  // e-ink: do full refresh every n partial refreshes (0=never)
+  virtual void forceFullRefresh() { }  // e-ink: make the next frame a full (non-partial) refresh — clears ghosting on screen change; no-op on OLED
   virtual void endFrame() = 0;
 
 #ifdef ENABLE_SCREENSHOT