refactor: rename firmware from Plus to Solo

- FIRMWARE_VERSION: v1.15-plus.1 → v1.15-solo.1
- FIRMWARE_PLUS_BUILD → FIRMWARE_SOLO_BUILD in all platformio.ini and UITask.cpp
- docs/plus_features/ → docs/solo_features/ (all sub-docs moved)
- README: update all doc links plus new Documentation section
- GitHub Actions: extract GIT_TAG_VERSION in each build job (was missing,
  causing empty FIRMWARE_VERSION in builds); rename output files to
  solo-VERSION-oled.uf2 / solo-VERSION-eink.uf2; release title → "Solo VERSION"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/solo_features/screen_lock/screen_lock.md

@@ -0,0 +1,51 @@
+## Screen Lock
+
+[Go back](../../../README.md)
+
+### Overview
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./lck_scr_1_oled.png) | ![](./lck_scr_1_eink.png) |
+
+Screen lock prevents accidental keypresses. While locked the display turns off and all input is ignored.
+
+---
+
+### Locking and unlocking
+
+**Hold Back** and press **Enter** three times within 3 seconds. The sequence works in both directions — the same combination locks and unlocks.
+
+If the display is off when the sequence begins, it turns on automatically so the hint is visible. Each press in the sequence extends the display-on timer by 5 seconds.
+
+The hint popup at the bottom of the lock screen guides through the sequence:
+
+| Step           | Hint                  |
+| -------------- | --------------------- |
+| Not started    | _Hold Back + 3×Enter_ |
+| 1 press done   | _Enter ×2 more…_      |
+| 2 presses done | _Enter ×1 more…_      |
+
+If no press is made for 3 seconds, the counter resets.
+
+---
+
+### Lock screen
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./lck_scr_2_oled.png) | ![](./lck_scr_2_eink.png) |
+
+A brief press of any button wakes the display and shows the lock screen. It displays:
+
+- **Time** — large, same format as the Clock page (24 h / 12 h from Settings)
+- **Date** — day-of-week, day, month
+- **Two sensor values** — the first two Dashboard Config fields (same values configured for the Clock page); shown side by side if both are set
+
+The display turns off again automatically after 5 seconds of inactivity (or 2 seconds immediately after locking).
+
+---
+
+### Auto-lock
+
+Enable **Auto-lock** in **Settings › Display** to lock the device automatically whenever the display turns off due to auto-off timeout. With auto-lock on, the device is always locked after the screen goes dark — no manual lock needed.