v1.9.1: PNG Material Design icons — 28 icons, dark/light theme, HiDPI, graceful Unicode fallback

- 56 PNG icons (28 unique × 2 color variants) from Material Design Icons (round style, 96×96px)
- core/icons.py: ctk_icon(), make_icon_button(), reconfigure_icon_button() with CTkImage cache
- Updated 15 GUI files: app.py, sidebar.py, server_dialog.py, all tabs
- build.py: auto-include assets/icons/ in PyInstaller bundle, patch rollover at 99→minor+1
- tools/download_icons.py: icon download script
- Automatic dark↔light theme switching via CTkImage dual-image support

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

gui/tabs/query_tab.py

@@ -14,7 +14,7 @@ from tkinter import ttk, filedialog
 import customtkinter as ctk
 
 from core.i18n import t
-from core.icons import icon_text
+from core.icons import icon_text, make_icon_button
 from core.sql_client import SQLClient
 
 _TREE_THEME_APPLIED = False
@@ -227,9 +227,8 @@ class QueryTab(ctk.CTkFrame):
         btn_row = ctk.CTkFrame(right_ctk, fg_color="transparent")
         btn_row.pack(fill="x", padx=8, pady=4)
 
-        self._exec_btn = ctk.CTkButton(
-            btn_row,
-            text=icon_text("execute", t("query_execute")),
+        self._exec_btn = make_icon_button(
+            btn_row, "execute", t("query_execute"),
             command=self._execute_query,
             width=130,
             fg_color="#2563eb",
@@ -237,9 +236,8 @@ class QueryTab(ctk.CTkFrame):
         )
         self._exec_btn.pack(side="left", padx=(0, 6))
 
-        self._clear_btn = ctk.CTkButton(
-            btn_row,
-            text=icon_text("clear", t("query_clear")),
+        self._clear_btn = make_icon_button(
+            btn_row, "clear", t("query_clear"),
             command=self._clear_all,
             width=80,
             fg_color="#6b7280",
@@ -247,9 +245,8 @@ class QueryTab(ctk.CTkFrame):
         )
         self._clear_btn.pack(side="left", padx=(0, 6))
 
-        self._export_btn = ctk.CTkButton(
-            btn_row,
-            text=icon_text("save", t("query_export_csv")),
+        self._export_btn = make_icon_button(
+            btn_row, "save", t("query_export_csv"),
             command=self._export_csv,
             width=110,
             fg_color="#059669",