feat(ui-core,ui-lvgl): channel management in the Core, L2 channel screens

ChannelControl (add Public/Hashtag/Private, rename, delete, notif/fav/scope)
and Favourites slots move into ui-core; ui-new uses them. Contact / channel
pref cleanup moves from ui-new's UITask into UiCore so it runs on every
frontend (L2 had none). L2: channel options popup (hold a row or the gear
in a channel) and the Add / Edit channel form.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/design/solo_ui_framework.md

@@ -358,8 +358,9 @@ with `resolvePersonPos()` (live `[LOC]` share, else last-advertised fix).
   (the ringtone player moved to a hardware timer for this reason).
 - **Reference cleanup.** Anything that remembers a contact by pubkey (favourite
   slot, Locator/Live-Share target, per-contact mute/melody) or a channel by
-  index must drop that reference when the entity goes away — hook
-  `UITask::onContactRemoved()` / `onChannelRemoved()`. New per-contact or
+  index must drop that reference when the entity goes away. The UI Core does
+  it for every frontend: `UiCore::cleanupContactPrefs()` (ui-core/UiCore.h) and
+  `chanctl::onRemoved()` (ui-core/ChannelControl.h). New per-contact or
   per-channel state should clear there too.
 - **Toasts:** `_task->showAlert("msg", duration_ms)` overlays a transient banner
   over any screen; no redraw plumbing needed.