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/models/connection_model.py

@@ -14,7 +14,7 @@ class ConnectionProfile:
     database: str = ""
     username: str = ""
     password: str = field(default="", repr=False)
-    color: str = "#89b4fa"
+    color: str = "#8caaee"
     id: str = field(default_factory=lambda: str(uuid.uuid4()))
     ssl: bool = False
     ssl_cert: str = ""
@@ -71,7 +71,7 @@ class ConnectionProfile:
             port=data.get("port", 3306),
             database=data.get("database", ""),
             username=data.get("username", ""),
-            color=data.get("color", "#89b4fa"),
+            color=data.get("color", "#8caaee"),
             ssl=data.get("ssl", False),
             ssl_cert=data.get("ssl_cert", ""),
             ssl_key=data.get("ssl_key", ""),

app/models/result_table_model.py

@@ -11,7 +11,7 @@ from PyQt6.QtGui import QColor, QFont
 class ResultTableModel(QAbstractTableModel):
     """Immutable result-set model — replaces data via set_data()."""
 
-    NULL_COLOR  = QColor("#6c7086")   # muted grey for NULL
+    NULL_COLOR  = QColor("#737994")   # muted grey for NULL
     NUM_ALIGN   = Qt.AlignmentFlag.AlignRight | Qt.AlignmentFlag.AlignVCenter
     TEXT_ALIGN  = Qt.AlignmentFlag.AlignLeft  | Qt.AlignmentFlag.AlignVCenter