fix(config): fix empty General tab — height collapse and invalid CSS variable

_GeneralTab and _PluginConfigTab inherited from Widget (height: auto), causing
the inner VerticalScroll to get height: 1fr of an auto-height parent, which
collapsed to 0. Fix: inherit from VerticalScroll directly and remove the inner
wrapper. _PluginsTab gets DEFAULT_CSS to fill its TabPane.

Also replace $text-muted (invalid in Textual 8.x) with $foreground 50%.

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

src/pyra/config/tui.py

@@ -67,10 +67,9 @@ def _pfid(plugin_name: str, key: str) -> str:
 
 # ── Tab widgets ───────────────────────────────────────────────────────────────
 
-class _GeneralTab(Widget):
+class _GeneralTab(VerticalScroll):
     def compose(self) -> ComposeResult:
         cfg = load_config()
-        with VerticalScroll():
         for f in GENERAL_FIELDS:
             current = _get_nested(cfg, f.path)
             with Horizontal(classes="row"):
@@ -99,6 +98,8 @@ class _GeneralTab(Widget):
 
 
 class _PluginsTab(Widget):
+    DEFAULT_CSS = "_PluginsTab { height: 1fr; width: 1fr; }"
+
     def compose(self) -> ComposeResult:
         cfg = load_config()
         enabled = set(cfg.plugins.enabled)
@@ -150,7 +151,7 @@ class _PluginsTab(Widget):
             )
 
 
-class _PluginConfigTab(Widget):
+class _PluginConfigTab(VerticalScroll):
     def __init__(self, name: str, plugin: Any) -> None:
         super().__init__()
         self._name = name
@@ -159,7 +160,6 @@ class _PluginConfigTab(Widget):
     def compose(self) -> ComposeResult:
         cfg = load_config()
         settings = cfg.plugin_settings.get(self._name, {})
-        with VerticalScroll():
         for f in self._plugin.config_fields():
             current = settings.get(f.key, f.default)
             with Horizontal(classes="row"):
@@ -202,7 +202,7 @@ class ConfigApp(App):
     Screen { background: $surface; }
     .row { height: 3; margin: 0 2; }
     .row Label { width: 26; content-align: left middle; }
-    .hint { color: $text-muted; margin: 0 2 1 28; }
+    .hint { color: $foreground 50%; margin: 0 2 1 28; }
     .actions { height: 3; align: right middle; margin: 1 2; }
     DataTable { height: 1fr; }
     """