fix(tui): refresh API key placeholder when switching providers

When the user switches providers in the AI tab, the key Input now shows
"set" or "not set" based on what's actually stored in the vault for that
provider, and clears any in-progress key entry.

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

src/pyra/config/tui.py

@@ -149,6 +149,14 @@ class _AITab(VerticalScroll):
         show_key = provider.requires_key
         self.query_one("#ai-key-row").display = show_key
         self.query_one("#ai-key-hint").display = show_key
+        if show_key:
+            try:
+                has_key = get_key(provider_id) is not None
+            except Exception:
+                has_key = False
+            key_input = self.query_one("#ai-key", Input)
+            key_input.placeholder = "set" if has_key else "not set"
+            key_input.value = ""
 
     def action_save(self) -> None:
         self._do_save()