fix: resolve mypy type errors in QSettings and QApplication usage

QSettings.value() returns `object` in PySide6 stubs — cast with
int(str(...)) for numeric values and bool(...) for booleans.

QApplication.instance() returns QCoreApplication in stubs, but
setStyle/setPalette are QApplication-only — use cast(QApplication, ...)
to restore correct type narrowing.

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

Author: chrisdpurcell

src/views/main_window.py

@@ -596,7 +596,7 @@ def _apply_preferences(self) -> None:
 
         # Font
         family = str(settings.value(KEY_FONT_FAMILY, DEFAULTS[KEY_FONT_FAMILY]))
-        size = int(settings.value(KEY_FONT_SIZE, DEFAULTS[KEY_FONT_SIZE]))
+        size = int(str(settings.value(KEY_FONT_SIZE, DEFAULTS[KEY_FONT_SIZE])))
         self._plain_text_edit.setFont(QFont(family, size))
 
         # Word wrap
@@ -609,7 +609,7 @@ def _apply_preferences(self) -> None:
 
         # Theme — Fusion style with a dark palette; restore system default for light.
         theme = str(settings.value(KEY_THEME, DEFAULTS[KEY_THEME]))
-        app = QApplication.instance()
+        app = cast(QApplication, QApplication.instance())
         if app is not None:
             if theme == "dark":
                 app.setStyle("Fusion")

src/views/preferences_dialog.py

@@ -141,13 +141,15 @@ def _load_from_settings(self) -> None:
             str(settings.value(KEY_FONT_FAMILY, DEFAULTS[KEY_FONT_FAMILY]))
         )
         self._font_size_spin.setValue(
-            int(settings.value(KEY_FONT_SIZE, DEFAULTS[KEY_FONT_SIZE]))
+            int(str(settings.value(KEY_FONT_SIZE, DEFAULTS[KEY_FONT_SIZE])))
         )
         self._word_wrap_cb.setChecked(
-            settings.value(KEY_WORD_WRAP, DEFAULTS[KEY_WORD_WRAP], type=bool)
+            bool(settings.value(KEY_WORD_WRAP, DEFAULTS[KEY_WORD_WRAP], type=bool))
         )
         self._line_numbers_cb.setChecked(
-            settings.value(KEY_LINE_NUMBERS, DEFAULTS[KEY_LINE_NUMBERS], type=bool)
+            bool(
+                settings.value(KEY_LINE_NUMBERS, DEFAULTS[KEY_LINE_NUMBERS], type=bool)
+            )
         )
         theme = settings.value(KEY_THEME, DEFAULTS[KEY_THEME])
         self._theme_dark_radio.setChecked(theme == "dark")