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/favourites_dial/favourites_dial.md

@@ -0,0 +1,59 @@
+## Favourites Dial
+
+[Go back](../../../README.md)
+
+### Overview
+
+|            OLED            |           E-Ink            |
+| :------------------------: | :------------------------: |
+| ![](./fav_dial_1_oled.png) | ![](./fav_dial_1_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:
+
+- **Portrait** (OLED, e-ink portrait) — 2 columns × 3 rows
+- **Landscape** (e-ink landscape) — 3 columns × 2 rows
+
+---
+
+### Navigation
+
+Navigate tiles with **UP / DOWN / LEFT / RIGHT**. Pressing a directional key at the edge of the grid switches to the adjacent home page instead of wrapping.
+
+**Enter on a filled tile** — opens that contact's DM directly.
+
+**Enter on an empty tile (`+`)** — opens a contact picker to fill the slot.
+
+---
+
+### Unread badge
+
+Filled tiles show an unread message count in the top-right corner when there are unread DMs from that contact. The contact name is ellipsized to make room for the badge.
+
+If a pinned contact has been removed from the contacts list, the tile shows `(gone)` until the slot is reassigned.
+
+---
+
+### Pinning a contact
+
+**From the Favourites Dial** — press **Enter** on an empty tile (`+`). A picker opens showing:
+
+1. Contacts marked as favourites in the upstream app (starred contacts) — listed first
+2. Recent DM contacts — listed after
+
+Select a contact to pin it to that slot.
+
+**From a DM conversation** — **Hold Enter** › context menu › **Pin to dial**, then choose a slot from the slot picker (Slot 1–6, showing the current occupant name or "empty").
+
+If the selected contact is already pinned in another slot, it is moved to the new slot automatically.
+
+---
+
+### Unpinning a contact
+
+Open the contact's DM, **Hold Enter** › context menu › **Unpin (slot N)**.
+
+---
+
+### Reordering the Favourites page
+
+The position of the Favourites Dial in the home page navigation sequence can be changed in **Settings › Home Pages** — press **LEFT / RIGHT** on the Favourites entry to move it earlier or later.