fix(mesh): route DM-via-channel on channel 0 (channel 1 unsupported)

Firmware rejected send_channel_text(1, ...) with "Unsupported command"
because channel 1 isn't configured on the device. Revert to channel 0
for the DM wrapper — the 0xD1 marker + dest_prefix header still
disambiguates DMs from plain public-channel text. Also revert
Mesh.vue publicChannel back to index 0 so user-typed broadcasts
target the same (only) working channel.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

neode-ui/src/views/Mesh.vue

@@ -41,7 +41,7 @@ let pollInterval: ReturnType<typeof setInterval> | null = null
 // + RF region. Slot 1 is set by archipelago first-boot to a hash derived
 // from the channel_name, so all archipelago nodes are guaranteed on the
 // same channel regardless of region.
-const publicChannel = { index: 1, name: 'Public' }
+const publicChannel = { index: 0, name: 'Public' }
 
 // Channel contact_id convention: matches backend u32::MAX - channel_index
 function channelContactId(channelIndex: number): number {