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/table_structure.py

@@ -171,9 +171,9 @@ class TableStructureView(QWidget):
                 item = QTableWidgetItem(str(val))
                 item.setTextAlignment(Qt.AlignmentFlag.AlignCenter)
                 if col.is_primary_key and c == 0:
-                    item.setForeground(QColor("#f9e2af"))
+                    item.setForeground(QColor("#e5c890"))
                 elif col.is_foreign_key and c == 0:
-                    item.setForeground(QColor("#89dceb"))
+                    item.setForeground(QColor("#99d1db"))
                 t.setItem(r, c, item)
 
     def _load_indexes(self):