Channel import shows generic 'Invalid channel link / key' when private-channel slots are full

[MrSurly]

Summary

ChannelRepository.importChannel() (used by the "Add channel via link/QR code" dialog) silently returns null when the per-companion private-channel index pool is exhausted, instead of surfacing a specific error. The UI then shows the generic "Invalid channel link / key" snackbar, which is misleading -- the link/key is valid, there's just no free channel slot.

This is not platform-specific. It reproduces on any platform once a companion already has _maxPrivateChannels (currently 7) private channels registered.

Where

lib/repositories/channel_repository.dart, importChannel():

• Computes nextIndex via _nextIndexFor(localOnly, companionKey) (line ~313).
• If no index is available, _nextIndexFor returns null, and importChannel just does if (nextIndex == null) return null; -- no exception, no distinguishing error.
• Contrast with createChannel() / createPrivateChannel() in the same file, which handle the identical "no free index" case by throwing a specific StateError('Maximum number of channels ($_maxPrivateChannels) reached').
• importChannel()'s blanket try { ... } catch (_) { return null; } also swallows firmware-registration failures (_registerChannelWithFirmware returning !isSuccess) the same way, whereas the create-channel paths surface result.errorCode (including a specific message for error code 3 / max channels).

Repro

1. Have a companion with 7 private (non-public) channels already registered (indices 1-7 all used).
2. Try to import an 8th channel via a meshcore://channel/add?name=...&secret=... link, either by pasting the full URL or scanning a QR code, in the "Add channel via link/QR code" dialog.
3. Observe: "Invalid channel link / key" snackbar, even though the link/secret are well-formed and decode correctly.

Expected

importChannel() should distinguish failure modes the same way createChannel/createPrivateChannel already do, and the UI should show a specific message like "Maximum number of channels reached" instead of the generic invalid-link message.

Suggested fix

Either:

• Have importChannel() throw the same StateErrors that createChannel/createPrivateChannel throw for max-channels-reached and firmware-registration failure, and let the dialog's existing catch (e) block (around lib/screens/channels_screen.dart) display e.toString(), or
• Change importChannel()'s return type to something that can carry a specific failure reason instead of ChannelData?.