theme: upgrade palette from Catppuccin Mocha to Frappé

Replaces all dark-Mocha colours (#1e1e2e base, #313244 surface0, etc.)
with the noticeably brighter Frappé variants (#303446 base, #414559
surface0, etc.) across the QSS and every hardcoded colour in Python
source files (syntax highlighter, completer popup, log viewer, explain
view, table viewer, schema browser, icons, etc.).

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

app/ui/connection_dialog.py

@@ -17,7 +17,7 @@ from app.config.connections import save_profile
 class ColorButton(QPushButton):
     """A button that shows a solid colour and opens a colour picker."""
 
-    def __init__(self, color: str = "#89b4fa", parent=None):
+    def __init__(self, color: str = "#8caaee", parent=None):
         super().__init__(parent)
         self._color = color
         self.setFixedSize(32, 24)
@@ -26,7 +26,7 @@ class ColorButton(QPushButton):
 
     def _refresh(self):
         self.setStyleSheet(
-            f"background-color:{self._color}; border:1px solid #45475a; border-radius:4px;"
+            f"background-color:{self._color}; border:1px solid #51576d; border-radius:4px;"
         )
 
     def _pick(self):