docs: rename screenshots to descriptive names

Switch the screenshot convention from positional numbers (tls_scr_8) to
self-documenting names (locator, locator_picker, map_target, …). Names say what
they show, so a shot can be re-captured or inserted without renumbering, and a
reference can't silently point at the wrong image. Per-doc directories keep the
names unambiguous, so no prefix is needed (overview_oled.png lives in each
screen's own folder).

git mv on every existing PNG + ref updates across all six solo_features docs;
the pending placeholders adopt the same scheme.

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

docs/solo_features/favourites_dial/favourites_dial.md

@@ -6,7 +6,7 @@
 
 |            OLED            |           E-Ink            |
 | :------------------------: | :------------------------: |
-| ![](./fav_dial_1_oled.png) | ![](./fav_dial_1_eink.png) |
+| ![](./overview_oled.png) | ![](./overview_eink.png) |
 
 A dedicated home page showing a grid of up to 6 pinned contacts for quick access. The layout adapts to the display orientation:
 
@@ -43,7 +43,7 @@ If a pinned contact has been removed from the contacts list, the tile shows `(go
 
 |            OLED            |           E-Ink            |
 | :------------------------: | :------------------------: |
-| ![](./fav_dial_2_oled.png) | ![](./fav_dial_2_eink.png) |
+| ![](./picker_oled.png) | ![](./picker_eink.png) |
 
 <!-- screenshot pending: Pin-to-dial contact picker (favourites first, then recent DMs, then all contacts) -->