feat(omemo): suppress repetitive missing device ID warnings

When sending OMEMO messages, we print a warning message for every
participant without a device ID on every single send:

```
Can't find a OMEMO device id for my@jid.org
```

This clutters the UI due to the high volume of repetitive information.

This is solved by tracking suppressed warnings within the window
structure. A warning for a specific JID and type is only shown once
per window context.

Suppression state is stored in the base ProfWin structure and managed
via helper functions (win_warn_needed, win_warn_sent). The state is
lazily initialized to save resources and automatically cleaned up when
the window is closed.

Warnings are explicitly reset when an OMEMO session is started or
ended to ensure users see fresh alerts when toggling encryption.

tests/unittests/ui/stub_ui.c

@@ -1318,6 +1318,22 @@ win_refresh_with_subwin(ProfWin* window)
 {
 }
 
+gboolean
+win_warn_needed(ProfWin* window, const char* const type, const char* const jid)
+{
+    return TRUE;
+}
+
+void
+win_warn_sent(ProfWin* window, const char* const type, const char* const jid)
+{
+}
+
+void
+win_clear_warned_jids(ProfWin* window)
+{
+}
+
 void
 win_println(ProfWin* window, theme_item_t theme, const char* ch, const char* const message, ...)
 {