fix(desktop): serialize tray channel actions for frontend (#4762)

## Summary
- serialize native `openChannel` tray actions with the camelCase field
names consumed by the TypeScript frontend
- prevent a valid tray channel ID from becoming `/channels/undefined`
- add a Rust serialization contract test covering the complete frontend
payload shape


### Root cause
`TrayAction` renamed the enum variant to `openChannel`, but its struct
fields still serialized as `channel_id` and `community_generation`. The
frontend reads `action.channelId`, so tray navigation called
`goChannel(undefined)`.


### Testing
- manually verified the corrected runtime payload and tray navigation
before removing temporary logging
- `just desktop-ci`
- pre-push hooks (desktop checks/tests, Tauri checks, and Rust tests)

---------

Signed-off-by: Kalvin Chau <kalvin@block.xyz>
Signed-off-by: npub1dccv64krpcpse5cmkzfeh998cftungyatw3djt8jwdw6g43f7fyqzzmrf7 <6e30cd56c30e030cd31bb0939b94a7c257c9a09d5ba2d92cf2735da45629f248@buzz.block.builderlab.xyz>
Co-authored-by: npub122y0pqkertljmedu303rl0aqrj3w8pvu43t6jxm6875lzg6f2pwqegc3xc <5288f082d91aff2de5bc8be23fbfa01ca2e3859cac57a91b7a3fa9f12349505c@buzz.block.builderlab.xyz>
Co-authored-by: npub1dccv64krpcpse5cmkzfeh998cftungyatw3djt8jwdw6g43f7fyqzzmrf7 <6e30cd56c30e030cd31bb0939b94a7c257c9a09d5ba2d92cf2735da45629f248@buzz.block.builderlab.xyz>

desktop/src-tauri/src/tray_menu.rs

@@ -213,7 +213,9 @@ struct TrayMenuState<R: Runtime> {
 pub enum TrayAction {
     NewChannel,
     OpenChannel {
+        #[serde(rename = "channelId")]
         channel_id: String,
+        #[serde(rename = "communityGeneration")]
         community_generation: u64,
     },
 }
@@ -614,6 +616,23 @@ pub fn update_tray_agent_activity<R: Runtime>(
 mod tests {
     use super::{requeue_actions, TrayAction, TrayActionQueue};
 
+    #[test]
+    fn open_channel_action_serializes_with_frontend_field_names() {
+        let action = TrayAction::OpenChannel {
+            channel_id: "channel-123".into(),
+            community_generation: 7,
+        };
+
+        assert_eq!(
+            serde_json::to_value(action).expect("tray action should serialize"),
+            serde_json::json!({
+                "kind": "openChannel",
+                "channelId": "channel-123",
+                "communityGeneration": 7,
+            })
+        );
+    }
+
     #[test]
     fn stale_channel_actions_are_not_requeued_after_community_change() {
         let mut queue = TrayActionQueue {