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/keys_tab.py

@@ -7,7 +7,7 @@ import threading
 import customtkinter as ctk
 from core.ssh_client import SSHClientWrapper
 from core.i18n import t
-from core.icons import icon_text
+from core.icons import icon_text, make_icon_button
 
 
 class KeysTab(ctk.CTkFrame):
@@ -30,13 +30,13 @@ class KeysTab(ctk.CTkFrame):
         btn_frame = ctk.CTkFrame(self, fg_color="transparent")
         btn_frame.pack(fill="x", padx=15, pady=5)
 
-        self.gen_btn = ctk.CTkButton(btn_frame, text=icon_text("key", t("generate_key")), command=self._generate)
+        self.gen_btn = make_icon_button(btn_frame, "key", t("generate_key"), command=self._generate)
         self.gen_btn.pack(side="left", padx=(0, 10))
 
-        self.install_btn = ctk.CTkButton(btn_frame, text=icon_text("upload", t("install_on_server")), fg_color="#22c55e", hover_color="#16a34a", command=self._install)
+        self.install_btn = make_icon_button(btn_frame, "upload", t("install_on_server"), fg_color="#22c55e", hover_color="#16a34a", command=self._install)
         self.install_btn.pack(side="left")
 
-        self.copy_btn = ctk.CTkButton(btn_frame, text=icon_text("copy", t("copy_public_key")), fg_color="#6b7280", command=self._copy_key)
+        self.copy_btn = make_icon_button(btn_frame, "copy", t("copy_public_key"), fg_color="#6b7280", command=self._copy_key)
         self.copy_btn.pack(side="right")
 
         # Status log