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/query_history.py

@@ -153,7 +153,7 @@ class QueryHistoryPanel(QWidget):
             for c, val in enumerate(items):
                 item = QTableWidgetItem(val)
                 if c == 3 and status == "ERROR":
-                    item.setForeground(QColor("#f38ba8"))
+                    item.setForeground(QColor("#e78284"))
                 self._table.setItem(r, c, item)
 
     def _filter(self, text: str):