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/app.py

@@ -11,7 +11,7 @@ from core.status_checker import StatusChecker
 from core.updater import UpdateChecker
 from core import i18n
 from core.i18n import t, LANGUAGES
-from core.icons import icon, TAB_ICONS
+from core.icons import icon, TAB_ICONS, ctk_icon
 from core.session_pool import SessionPool
 from gui.sidebar import Sidebar
 from gui.server_dialog import ServerDialog
@@ -147,7 +147,11 @@ class App(ctk.CTk):
         header_bar.pack_propagate(False)
 
         # Language selector
-        self._lang_icon = ctk.CTkLabel(header_bar, text="\U0001f310", font=ctk.CTkFont(size=14), width=20)
+        _lang_img = ctk_icon("globe", 18)
+        self._lang_icon = ctk.CTkLabel(
+            header_bar, text="" if _lang_img else "\U0001f310",
+            image=_lang_img, font=ctk.CTkFont(size=14), width=20,
+        )
         self._lang_icon.pack(side="right", padx=(5, 0))
         lang_values = list(LANGUAGES.values())
         current_display = LANGUAGES.get(i18n.get_language(), "English")
@@ -159,16 +163,20 @@ class App(ctk.CTk):
         self.lang_menu.pack(side="right", padx=(5, 0))
 
         # Check Updates button
+        _sync_img = ctk_icon("refresh", 18)
         self._update_check_btn = ctk.CTkButton(
-            header_bar, text="\u21bb", width=30, height=30,
+            header_bar, text="" if _sync_img else "\u21bb",
+            image=_sync_img, width=30, height=30,
             corner_radius=15, fg_color="#6b7280", hover_color="#4b5563",
             command=self._check_updates_manual,
         )
         self._update_check_btn.pack(side="right", padx=(5, 0))
 
         # About button
+        _info_img = ctk_icon("info", 18)
         self.about_btn = ctk.CTkButton(
-            header_bar, text="ⓘ", width=30, height=30,
+            header_bar, text="" if _info_img else "ⓘ",
+            image=_info_img, width=30, height=30,
             corner_radius=15, fg_color="#6b7280", hover_color="#4b5563",
             command=self._show_about
         )