SSH client: a terminal on another machine (#2)

The SSH App opens one session to a shell, over libssh (LibSSH-ESP32 5.10.0)
on mbedTLS. A server is trusted the first time on its fingerprint, and a
changed key is a warning with Cancel selected. The password is typed each
time and kept nowhere; or the device makes itself an Ed25519 key, whose
public half is shown, written to /ssh/id_ed25519.pub and printed by
`ssh status`.

lib/term is the terminal: what a shell, less, top, nano and vim send, with
sixteen colours, scroll regions, the alternate screen and 100 lines of
scrollback. Five text sizes with Ctrl and + or -, from 60x20 to 26x8, told
to the far end. The session goes on when the App is left; SSH shows in the
Status Bar. `ssh user@host` in the Shell opens the App.

Also:
- Keys that aren't characters carry Shift, Ctrl and Alt. The terminal needs
  it, and it makes Ctrl+Fn+up/down in a note and Shift+Tab in Gemini work
  from the real keyboard.
- IRC doesn't try to connect under 60 KB free: started with a session open,
  its TLS handshake took the heap down to 236 bytes.
- libssh's own curve25519 is left out of the build (scripts/libssh_filter.py):
  libsodium's has the same names.

Costs 292 KB of flash and about 50 KB of heap while a session is open; not
started under 75 KB free.

Docs: guide page, how-to, FAQ, home page, Status Bar, SD card folders, the
memory how-to, README, glossary, N1 notes with Q254 to Q266 and the checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

site/content/guide/basics.md

@@ -49,6 +49,7 @@ A strip at the top of every screen: the name of the App on the left, and on the
 | `97%` | The battery (in the warning colour at 15% and under) |
 | `SD` | A card is in; the warning colour at 80% full |
 | bars and `W` | Wi-Fi connected, with its signal; `W?` is searching; `MON` is the Wi-Fi radio in its monitoring mode, which pauses IRC |
+| `SSH` | An [SSH session](/guide/ssh/) is open, whatever App is in front |
 | `VPN` | The [WireGuard tunnel](/guide/vpn/) is wanted; brighter once the server has answered |
 | `DBG` | The Debug Console is switched on (Settings → Debug Console); brighter while a PC is connected to it |
 | `REC` | A GNSS Track is being recorded |