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/schema_browser.py

@@ -96,7 +96,7 @@ class SchemaBrowser(QWidget):
         # Dim colour — show the profile colour but muted
         color = QColor(profile.color)
         color.setAlpha(140)
-        item.setForeground(0, QBrush(QColor("#6c7086")))   # greyed out
+        item.setForeground(0, QBrush(QColor("#737994")))   # greyed out
         item.setIcon(0, get_icon(profile.db_type, 16))
         item.setToolTip(0, self._connection_tooltip(profile))
 
@@ -350,9 +350,9 @@ class SchemaBrowser(QWidget):
             child = QTreeWidgetItem([label])
             child.setData(0, Qt.ItemDataRole.UserRole, (NT["column"], "", "", c.name))
             if c.is_primary_key:
-                child.setForeground(0, QBrush(QColor("#f9e2af")))
+                child.setForeground(0, QBrush(QColor("#e5c890")))
             elif c.is_foreign_key:
-                child.setForeground(0, QBrush(QColor("#89dceb")))
+                child.setForeground(0, QBrush(QColor("#99d1db")))
             item.addChild(child)
 
     def _load_indexes(self, item, pid, db, table):
@@ -386,7 +386,7 @@ class SchemaBrowser(QWidget):
 
     def _show_error(self, item, msg):
         err = QTreeWidgetItem([f"❌ {msg}"])
-        err.setForeground(0, QBrush(QColor("#f38ba8")))
+        err.setForeground(0, QBrush(QColor("#e78284")))
         item.addChild(err)
 
     # ── Context menu ──────────────────────────────────────────────────────────