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/utils/icons.py

@@ -64,7 +64,7 @@ ICONS = {
 
 
 def make_icon(glyph: str, size: int = 20,
-              fg: str = "#cdd6f4", bg: str = "transparent") -> QIcon:
+              fg: str = "#c6d0f5", bg: str = "transparent") -> QIcon:
     """Create a QIcon from a Unicode glyph."""
     px = QPixmap(QSize(size, size))
     px.fill(Qt.GlobalColor.transparent)