fix(#333): load channels before channel history, or history reads empty

Channel history is keyed by the channel's PSK (#194), and the resolver set on
the store reads that PSK out of _channels. After SELF_INFO, loadCachedChannels()
and loadAllChannelMessages() were both fired without awaiting, so the messages
could load while _channels was still empty. The resolver then returned null,
_storageKey() silently fell back to the legacy slot-index key, and channels
read as empty even though their history was intact under the PSK key.

Silent because the fallback is a legitimate code path: there is no error, no
parse failure and no log line, just an empty list. It is indistinguishable
from data loss to the user.

Observed on a live radio: Public held 566 messages under
channel_messages_<dev>psk_8b3387e9..., and the app displayed nothing.

loadAllChannelMessages() now runs only after loadCachedChannels() completes.

The startup call in main.dart already awaits in the right order; it runs before
any radio is connected, which is the source of the "Public key hex is not set"
warnings at launch, and is harmless once this post-connect load is correct.

Was previously committed against #291 on the nav-redesign branch; split out
here under its own issue.

lib/connector/meshcore_connector.dart

@@ -4646,10 +4646,14 @@ class MeshCoreConnector extends ChangeNotifier {
     _loadChannelOrder();
     loadContactCache();
     loadChannelSettings();
-    loadCachedChannels();
 
-    // Load persisted channel messages
+    // Channel history is keyed by the channel's PSK (#194), and the resolver
-    loadAllChannelMessages();
+    // set above reads that PSK out of _channels. So the channel list MUST be
+    // loaded before the messages are: loading them concurrently leaves the
+    // resolver returning null, _storageKey() silently falls back to the legacy
+    // slot-index key, and every channel reads as empty even though its history
+    // is intact under the PSK key.
+    unawaited(loadCachedChannels().then((_) => loadAllChannelMessages()));
     loadUnreadState();
     _loadDiscoveredContactCache();