fix: add photo_download_rx channel and fix account switcher nav tests

Add UnboundedReceiver for background photo downloads to App state,
reset it on close_chat. Fix account_switcher tests to navigate past
all accounts dynamically instead of assuming single account.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/account_switcher.rs

@@ -47,8 +47,15 @@ fn test_account_switcher_navigate_down() {
     let mut app = TestAppBuilder::new().build();
     app.open_account_switcher();
 
-    // Initially at 0, navigate down to "Add account" item
-    app.account_switcher_select_next();
+    let num_accounts = match &app.account_switcher {
+        Some(AccountSwitcherState::SelectAccount { accounts, .. }) => accounts.len(),
+        _ => panic!("Expected SelectAccount state"),
+    };
+
+    // Navigate down past all accounts to "Add account" item
+    for _ in 0..num_accounts {
+        app.account_switcher_select_next();
+    }
 
     match &app.account_switcher {
         Some(AccountSwitcherState::SelectAccount {
@@ -56,7 +63,7 @@ fn test_account_switcher_navigate_down() {
             accounts,
             ..
         }) => {
-            // Should be at index 1 (the "Add account" item, since default config has 1 account)
+            // Should be at the "Add account" item (index == accounts.len())
             assert_eq!(*selected_index, accounts.len());
         }
         _ => panic!("Expected SelectAccount state"),
@@ -116,8 +123,15 @@ fn test_confirm_add_account_transitions_to_add_state() {
     let mut app = TestAppBuilder::new().build();
     app.open_account_switcher();
 
-    // Navigate to "+ Add account"
-    app.account_switcher_select_next();
+    let num_accounts = match &app.account_switcher {
+        Some(AccountSwitcherState::SelectAccount { accounts, .. }) => accounts.len(),
+        _ => panic!("Expected SelectAccount state"),
+    };
+
+    // Navigate past all accounts to "+ Add account"
+    for _ in 0..num_accounts {
+        app.account_switcher_select_next();
+    }
 
     // Confirm should transition to AddAccount
     app.account_switcher_confirm();