docs: expand message_screen.md — all options, context menus, placeholders

- Describe DM/Channels/Rooms mode-select
- Describe MSG_PICK flow (keyboard vs quick replies Q1-Q10)
- Complete placeholder table — remove {batt} (shows in picker only
  when a voltage sensor is connected, not from board battery)
- Separate contact and channel context menus with full item tables
- Document LEFT/RIGHT cycling for Notif/Melody/Fav
- Describe Pin to dial with slot picker detail
- Clarify short Enter (fullscreen) vs Hold Enter (context menu)
- Add fullscreen navigation and scroll description

docs/plus_features/message_screen/message_screen.md

@@ -0,0 +1,106 @@
+## Messages Screen
+[Go back](../README.md)
+
+### Overview
+
+| OLED | E-Ink |
+|:----:|:-----:|
+| ![](./mes_scr_1_oled.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) | |
+
+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 (empty 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) | |
+
+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) | |
+
+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.
+
+**Hold Enter** in fullscreen opens the Reply option.
+
+---
+
+### Context menu — contact list
+
+| OLED | E-Ink |
+|:----:|:-----:|
+| ![](./mes_scr_5_oled.png) | |
+
+**Hold Enter** on a contact entry opens a context menu:
+
+| 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_6_oled.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.