feat: add branding assets and favicon support across admin-web and desktop-client

Add NexaVPN logo images (full logo and mark-only variants) to admin-web and desktop-client public directories. Add favicon.ico and favicon.png to admin-web, and icon.png to desktop-client. Update index.html files to reference favicon assets. Add icon.png and icon.ico to desktop-client Tauri icons directory and configure bundle.icon in tauri.conf.json. Update Layout component to display logo in sidebar brand-block with

scripts/generate_ico_from_png.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+from __future__ import annotations
+
+import struct
+import sys
+from pathlib import Path
+
+
+PNG_SIGNATURE = b"\x89PNG\r\n\x1a\n"
+
+
+def main() -> int:
+    if len(sys.argv) != 3:
+        print("usage: generate_ico_from_png.py <input.png> <output.ico>", file=sys.stderr)
+        return 1
+
+    source = Path(sys.argv[1])
+    target = Path(sys.argv[2])
+
+    png = source.read_bytes()
+    if not png.startswith(PNG_SIGNATURE):
+        print(f"{source} is not a PNG file", file=sys.stderr)
+        return 1
+
+    width = int.from_bytes(png[16:20], "big")
+    height = int.from_bytes(png[20:24], "big")
+    width_byte = 0 if width >= 256 else width
+    height_byte = 0 if height >= 256 else height
+
+    header = struct.pack("<HHH", 0, 1, 1)
+    directory = struct.pack(
+        "<BBBBHHII",
+        width_byte,
+        height_byte,
+        0,
+        0,
+        1,
+        32,
+        len(png),
+        len(header) + 16,
+    )
+
+    target.parent.mkdir(parents=True, exist_ok=True)
+    target.write_bytes(header + directory + png)
+    print(f"wrote {target}")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())