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/clock_screen/clock_screen.md

@@ -0,0 +1,55 @@
+## Clock Screen
+
+[Go back](../../../README.md)
+
+### Overview
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./clk_scr_1_oled.png) | ![](./clk_scr_1_eink.png) |
+
+A full-screen clock page on the home screen. Shows the current time and date, with up to three configurable data fields below.
+
+Time is synchronized from GPS or via the companion app. Timezone offset is applied from **Settings › System**.
+
+If no time source is available, the screen shows _"! No time sync"_ with a hint to enable GPS or connect the app.
+
+---
+
+### Time display
+
+- **Format** — 24 h or 12 h with AM/PM; configurable in **Settings › Display**
+- **Seconds** — shown by default on OLED; hidden on e-ink (always) and optionally on OLED via **Settings › Display › Clock seconds**; hiding reduces the refresh rate from 1 s to 60 s
+
+---
+
+### Data fields
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./clk_scr_2_oled.png) | ![](./clk_scr_2_eink.png) |
+
+Up to three data fields are shown below the date separator. Each field displays a label and a value on the same line.
+
+| Field       | Label | Value                                                                     |
+| ----------- | ----- | ------------------------------------------------------------------------- |
+| None        | —     | —                                                                         |
+| Batt V      | Batt  | Battery voltage (e.g. `3.92V`)                                            |
+| Batt %      | Batt  | Battery percentage using LiPo curve anchored at the low-battery threshold |
+| Temperature | Temp  | °C from onboard sensor                                                    |
+| Humidity    | Hum   | % from onboard sensor                                                     |
+| Pressure    | Pres  | hPa from onboard sensor                                                   |
+| GPS         | GPS   | `lat lon` decimal degrees, or `no fix`                                    |
+| Altitude    | Alt   | metres from GPS                                                           |
+| Luminosity  | Lux   | lux from onboard sensor                                                   |
+| CO₂         | CO2   | ppm from onboard sensor                                                   |
+| Contacts    | Nodes | Total contacts in the mesh                                                |
+| Messages    | Msgs  | Total unread message count                                                |
+
+Sensor fields show `--` when the sensor is not connected or has no data.
+
+---
+
+### Configuring fields
+
+**Hold Enter** (or press the **Context menu** key) on the Clock page to open the Dashboard Config screen, where each of the three field slots can be cycled with **LEFT/RIGHT**.