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/screen_lock/screen_lock.md

@@ -6,7 +6,7 @@
 
 |           OLED            |           E-Ink           |
 | :-----------------------: | :-----------------------: |
-| ![](./lck_scr_1_oled.png) | ![](./lck_scr_1_eink.png) |
+| ![](./overview_oled.png) | ![](./overview_eink.png) |
 
 Screen lock prevents accidental keypresses. While locked the display turns off and all input is ignored.
 
@@ -34,7 +34,7 @@ If no press is made for 3 seconds, the counter resets.
 
 |           OLED            |           E-Ink           |
 | :-----------------------: | :-----------------------: |
-| ![](./lck_scr_2_oled.png) | ![](./lck_scr_2_eink.png) |
+| ![](./screen_oled.png) | ![](./screen_eink.png) |
 
 A brief press of any button wakes the display and shows the lock screen. It displays: