docs: add a solo UI framework developer guide

Document the reusable building blocks the solo UI has grown — the UIScreen
model and screen wiring, DisplayDriver layout metrics, drawList/AccordionList,
the popup/keyboard/digit-editor/fullscreen/nav components, the geo + state-store
+ persistence helpers, mini-icons, input conventions, and the cross-cutting
gotchas (single-thread render, e-ink pacing, reference cleanup). Ends with a
worked "add a new Tools screen" example.

No code change; the helper layer is already well-factored (the recent dedup
passes closed the remaining gaps), so this captures it for contributors rather
than extracting more. Linked from the README docs table.

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

README.md

@@ -100,6 +100,7 @@ Updating to a newer version usually does not require erasing flash unless the re
 | [Settings Screen](./docs/solo_features/settings_screen/settings_screen.md) | All settings sections with values and interactions                    |
 | [Screen Lock](./docs/solo_features/screen_lock/screen_lock.md)             | Lock/unlock sequence, lock screen, auto-lock                          |
 | [Tools Screen](./docs/solo_features/tools_screen/tools_screen.md)          | GPS trail & waypoints, compass, navigation, nearby nodes, ringtone editor, auto-reply bot, auto-advert, live location sharing, locator, diagnostics, repeater |
+| [Solo UI framework](./docs/design/solo_ui_framework.md)                    | **Developer guide** — the reusable building blocks (screens, lists, popups, mini-icons, geo/persistence helpers) and how to add a new feature |
 
 ### Upstream MeshCore