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/message_screen/message_screen.md

@@ -0,0 +1,108 @@
+## Messages Screen
+
+[Go back](../../../README.md)
+
+### Overview
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./mes_scr_1_oled.png) | ![](./mes_scr_1_eink.png) |
+
+The Messages screen is split into three modes — **DMs**, **Channels**, and **Rooms** — selectable with UP/DOWN on the mode-select screen. Each mode shows the corresponding list of conversations with unread counters.
+
+---
+
+### Sending messages
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./mes_scr_2_oled.png) | ![](./mes_scr_2_eink.png) |
+
+Press **Enter** on a contact or channel to open its history, then press **Enter** again (or select an empty send row) to compose a message. Choose between:
+
+- **Custom message** — opens the on-screen keyboard
+- **Q1–Q10** — quick reply templates editable in Settings › Messages
+
+The keyboard supports placeholders that insert live data at send time:
+
+| Placeholder | Value                | Availability                |
+| ----------- | -------------------- | --------------------------- |
+| `{time}`    | current time (HH:MM) | always                      |
+| `{loc}`     | GPS coordinates      | always ("no GPS" if no fix) |
+| `{temp}`    | temperature          | sensor connected            |
+| `{hum}`     | humidity             | sensor connected            |
+| `{pres}`    | barometric pressure  | sensor connected            |
+| `{alt}`     | altitude             | sensor connected            |
+| `{lux}`     | luminosity           | sensor connected            |
+| `{co2}`     | CO₂ concentration    | sensor connected            |
+
+Sensor placeholders appear automatically in the placeholder picker when the corresponding sensor is active. `{time}` and `{loc}` are always shown.
+
+---
+
+### Message history
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./mes_scr_3_oled.png) | ![](./mes_scr_3_eink.png) |
+
+Each entry in the history list shows the sender name and a compact age indicator (`3m`, `2h`, `>1d`) in the top-right corner.
+
+**Short Enter** on a message opens it in fullscreen. **Hold Enter** opens the context menu (Reply option).
+
+---
+
+### Fullscreen message view
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./mes_scr_4_oled.png) | ![](./mes_scr_4_eink.png) |
+
+Navigate between messages with **LEFT** (newer) and **RIGHT** (older). Long messages scroll with **UP/DOWN**.
+
+If the message is a reply addressed to someone (`@[nick]`), a **To: nick** bar is shown below the sender name and the body is displayed without the address prefix.
+| OLED | E-Ink |
+|:----:|:-----:|
+| ![](./mes_scr_5_oled.png) |![](./mes_scr_5_eink.png) |
+**Hold Enter** in fullscreen opens the Reply option.
+
+---
+
+### Context menu — contact list
+
+**Hold Enter** on a contact entry opens a context menu:
+| OLED | E-Ink |
+|:----:|:-----:|
+| ![](./mes_scr_6_oled.png) | ![](./mes_scr_6_eink.png) |
+
+| Item                         | Action                                                                         |
+| ---------------------------- | ------------------------------------------------------------------------------ |
+| Mark as read                 | Clears unread counter for this contact                                         |
+| Notif: default / OFF / ON    | Per-contact notification override — **LEFT/RIGHT** to cycle                    |
+| Melody: global / M1 / M2     | Per-contact melody override — **LEFT/RIGHT** to cycle                          |
+| Pin to dial / Unpin (slot N) | Pin this contact to a Favourites Dial slot; if already pinned shows which slot |
+
+When **Pin to dial** is selected, a slot picker opens (Slot 1–6 showing current occupant name or "empty"). Choosing a slot that already holds another contact moves the new contact there.
+
+---
+
+### Context menu — channel list
+
+|           OLED            |           E-Ink           |
+| :-----------------------: | :-----------------------: |
+| ![](./mes_scr_7_oled.png) | ![](./mes_scr_7_eink.png) |
+
+**Hold Enter** on a channel entry opens a context menu:
+
+| Item                      | Action                                                                |
+| ------------------------- | --------------------------------------------------------------------- |
+| Mark all read             | Clears all unread for this channel                                    |
+| Notif: default / OFF / ON | Per-channel notification override — **LEFT/RIGHT** to cycle           |
+| Melody: global / M1 / M2  | Per-channel melody override — **LEFT/RIGHT** to cycle                 |
+| Fav: yes / no             | Add or remove this channel from favourites — **LEFT/RIGHT** to toggle |
+
+---
+
+### Mark all read
+
+**Hold Enter** on the DM / Channels / Rooms mode-select screen to clear all unread counters for the highlighted category at once.