feat(ui): channel Add gets a Type picker (Public/Hashtag/Private)

"+ Add channel" jumped straight into a generic Name+Secret form, so the
"hashtag channel" convention documented in docs/companion_protocol.md
(secret = first 16 bytes of sha256("#topic")) was only reachable by
already knowing to type a literal "#topic" into the passphrase field --
nothing in the UI surfaced it. The phone app instead shows an explicit
channel-type picker; this adds the same on-device.

"+ Add channel" now asks Public / Hashtag / Private first:
- Public commits immediately with the well-known default channel's name
  and secret (8b3387e9c5cdea6ac9e5edbaa115cd72, confirmed to match
  MyMesh.cpp's PUBLIC_GROUP_PSK and the docs' published key) -- useful
  to restore it if deleted.
- Hashtag shows a single Topic field; Save synthesizes name="#topic" and
  derives the secret via the existing SHA-256 passphrase path -- same
  underlying mechanism Private already had, just discoverable without
  knowing the "#" convention.
- Private is today's manual Name+Secret form, unchanged.

Editing an existing channel skips the picker (no ambiguity to resolve
there). Extracted hexToSecret() out of deriveSecret()'s hex-mode branch
so Public's fixed key parses through the same code instead of a second
hand-rolled loop. No other file needed changes -- openAdd()/openEdit()/
active()/render()/handleInput() keep their existing signatures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

release-notes.md

@@ -3,7 +3,7 @@
 ### What's new
 
 - **Nodes is now the single node hub.** "Nearby Nodes" is renamed **Nodes** and gains on-device contact management — no phone app needed: **Hold Enter** on a row opens **Add contact** (for a scanned-but-unknown node), **Favourite/Unfavourite** (pins to the first free Favourites Dial slot, shown as a star in the list), and **Delete contact** (confirm first, defaults to Cancel). The filter strip (**LEFT/RIGHT**) is now a visible, wraparound tab carousel instead of a hidden mode. The old read-only **Recent adverts** home page is retired — its passively-heard nodes now simply show up in the Nodes list (All filter) — so there's one list instead of two.
-- **On-device channel management.** Messages › Channels gets a **"+ Add channel"** row and per-channel **Edit**/**Delete** — create or join a channel by typing either a passphrase (hashed to the channel secret, same idea as a room password) or the exact 32-hex-character key from a channel QR code. No phone app required.
+- **On-device channel management.** Messages › Channels gets a **"+ Add channel"** row and per-channel **Edit**/**Delete** — no phone app required. Adding a channel now starts with a **type picker**, matching the phone app: **Public** (re-adds the well-known default channel, no typing needed), **Hashtag** (type just a topic name, e.g. `test` — the channel name and secret are both derived from it, same convention as the app's `#topic` channels), or **Private** (manual Name + Secret, typing either a passphrase hashed to the channel secret, or the exact 32-hex-character key from a channel QR code).
 - **Tools › Admin — administer a remote repeater/room from the device.** Log into a node you have admin rights on (same self-healing saved-password handshake as room logins) and browse its settings in category tabs — **System** (name, owner info), **Radio** (Frequency/Bandwidth/Spreading factor/Coding rate/TX power), **Routing** (Repeat/Advert interval/Flood advert interval/Max hops), **Actions** (reboot, send advert, sync clock, …) — plus a free-text **Custom command…** escape hatch (with **{}**-key command-name completion) for anything not covered. Radio and Routing fields use a type-appropriate editor instead of raw text — a digit-cursor for Frequency (the same widget Settings' own Radio screen uses locally), discrete-set stepping for Bandwidth/Spreading factor/Coding rate, plain number steppers for the rest, and an ON/OFF toggle for Repeat — adjusting a value costs no mesh traffic until you actually confirm it with **Enter** (**Cancel** sends nothing). Reachable either from **Tools › Admin** (opens the same Nodes list to pick a target) or directly via a repeater/room's own **Hold Enter › Admin** action.
 - **Auto-Reply Bot gains a third target: room servers**, alongside DM and Channel, with its own trigger/reply/commands — the screen is redesigned as a circular tab carousel (**Direct/Channel/Room/Other**, **LEFT/RIGHT** to switch, **UP/DOWN** within a tab) instead of one long list. Each target's **Enable** and **Commands** toggle is now fully independent (they used to secretly share the DM tab's setting for Channel/Room too). New `{name}`/`{hops}` reply placeholders, a DM allow-list (all chat contacts or favourites only), and a dedicated stepper for Quiet Hours.
 - **Keyboard: accent popup, selectable main script, better editing.** Holding **Enter** on a Latin letter with accented variants (`a c d e i l n o r s t u y z`) now opens a one-row popup of that letter's accents — e.g. holding `a` offers `á à â ã ä å ą` — covering every European Latin-diacritic letter (Polish, Czech, Slovak, German, French, Spanish, Portuguese, Nordic, …) from one popup instead of a separate page per language. **Settings › Keyboard** splits **Alphabet** into **Main** and **Additional**: Main picks which script (Latin/Cyrillic/Greek) the keyboard opens on by default, Additional picks the second one reached via the **#@/abc** cycle key — so a Cyrillic or Greek typist can make their own script the default instead of always landing on Latin first. Shift is now **one-shot** by default (capitalises just the next letter; **Hold Enter** on Shift toggles the old sticky caps-lock back on), **Hold Enter** on Backspace clears the whole field, and **UP** from the top letter row enters a cursor-positioning mode (**LEFT/RIGHT** move; **UP/DOWN** jump to start/end, then continue on to the special row / letter grid if pressed again once already there) so you can edit or insert anywhere in what you've typed, not just at the end.