Channels export to a file, and import onto another device

A new device ships with Public and nothing else, so changing devices
meant adding every channel again by hand: each private key pasted, each
# name typed, each send scope and mute set again. The Channels page now
has an "Export and import" section. Export writes every channel to one
TOML file, in slot order, with its key, its send scope and its mute.
Import reads such a file and adds its channels to the device.

Import never removes a channel. The channels of the file go first, in
the file's order, and whatever else the device holds follows them. A
channel with no free slot is reported, not forced in. Before anything is
written, a dialog shows the plan (what is added into which slot, what
moves) and ends on "Nothing is removed from the device." The slots are
read again in full before the plan is made, because a probe cut short
makes an occupied slot look free.

The file uses the same [[channels]] table as config backup, so a config
backup imports too. A # channel carries no secret, since the device
derives its key from the name. A private channel carries its key, so the
file is written readable by its owner only, and the export says how many
private keys it holds.

On the command line, `channels export FILE` writes the file and prints
its path, and `channels import --file FILE [--dry-run]` prints the
device's layout after the import, one row per channel with add, move,
keep or skip. `channels import INDEX URL` still imports a single link.

Refs: #25
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/cli/README.md

@@ -1281,6 +1281,8 @@ the room).
 | `add INDEX NAME [--secret HEX]` | Add a channel. A `#` at the start of the name makes the channel public (the firmware derives the key from the name). Otherwise the channel is private, with a random key unless you give a key. |
 | `join INDEX NAME SECRET` | Join a channel from its name and its key of 32 hex characters. |
 | `import INDEX URL` | Import a `meshcore://channel/add` link. |
+| `import --file FILE [--dry-run]` | Add the channels of a file from `export` to the device. |
+| `export FILE` | Write each channel to a file, with its key, its send scope, and its mute. Another device can import the file. |
 | `share INDEX` | Print the share link of a slot. |
 | `clear INDEX --yes` | Clear a slot. This removes the channel from the device. |
 | `scope INDEX [REGION] [--clear]` | Show or set the region into which the channel's messages are flooded. |
@@ -1340,6 +1342,55 @@ $ meshterm channels share 1 --json
 `share` on an empty slot, and `clear` on a slot that is already empty, give exit `5` with
 the nulls and `"cleared":false`. There is nothing to report, and it is not a failure.
 
+**Export and import.** `export` and `import --file` move all the channels to another
+device. `export` writes one TOML file, with one `[[channels]]` table for each channel, in
+slot order:
+
+```toml
+[[channels]]
+name = "#news"
+
+[[channels]]
+name = "brigade"
+secret = "2c70ca416304d520b60a28d9db35d6b3"
+scope = "harbour"
+muted = true
+```
+
+A `#` channel has no `secret`, because the device derives its key from the name. A private
+channel has its key in the file. Thus the file is as secret as the keys in it. MeshTerm
+makes it readable only by you, where the system lets it, and `export` says on stderr how
+many keys of private channels the file holds. A file from `config backup` has a
+`[[channels]]` table of the same shape, so `import --file` can also read a config backup.
+
+`import --file` never removes a channel. It puts the channels of the file first, in the
+order of the file, and the other channels of the device after them. A channel that has no
+free slot is not added. The send scope and the mute of each channel go into the stores of
+this computer. A channel that has no scope or no mute in the file keeps the values that it
+has here. `--dry-run` prints the plan and changes nothing.
+
+```console
+$ meshterm channels import --file brigade.toml
+SLOT  NAME     ACTION
+   0  #news    add
+   1  brigade  add
+```
+
+`import --file` prints the layout of the device after the import, with one record for each
+channel. `ACTION` is `add`, `move`, or `keep`. A channel that has no free slot comes last,
+with the action `skip` and no slot. `export` prints only the path of the file. It writes no
+file and exits with `5` when the device has no channel. A file that MeshTerm cannot use is
+a usage error (exit `2`), and the message names the bad entry. If MeshTerm cannot read
+each slot of the device, the command changes nothing and exits with `4`.
+
+```console
+$ meshterm channels import --file brigade.toml --json
+{"dry_run":false,"added":2,"moved":0,"skipped":0,"channels":[{"slot":0,"name":"#news","action":"add"},{"slot":1,"name":"brigade","action":"add"}]}
+
+$ meshterm channels export channels.toml --json
+{"path":null,"channels":0,"private":0}
+```
+
 #### `meshterm courier`
 
 A store-and-forward outbox. Queue a message for a contact that you cannot reach now. The