feat(vpn): VPN on-demand Tailscale + agent studioE5 standalone

- Agent studioE5 standalone en Go (console + systray)
- VPN on-demand via tailscaled + tailscale up (authkey Headscale)
- Resolver/serveur dans le tailnet studioe5
- Caddy on-demand TLS pour les instances
- Nouveaux endpoints serveur /api/internal/send-to-node
- Suppression des anciens binaires edubox-agent
- Suivi dans SUIVI_VPN_ONDEMAND.md

agent/generate_icon.py

@@ -0,0 +1,51 @@
+import zlib
+import struct
+
+
+def png_chunk(chunk_type: bytes, data: bytes) -> bytes:
+    return (
+        struct.pack(">I", len(data))
+        + chunk_type
+        + data
+        + struct.pack(">I", zlib.crc32(chunk_type + data) & 0xFFFFFFFF)
+    )
+
+
+def create_png(width: int, height: int, pixels: list) -> bytes:
+    """Create a PNG from a 2D list of (R, G, B) tuples."""
+    raw = bytearray()
+    for row in pixels:
+        raw.append(0)  # no filter
+        for r, g, b in row:
+            raw.extend((r, g, b))
+    compressed = zlib.compress(bytes(raw), 9)
+
+    ihdr_data = struct.pack(">IIBBBBB", width, height, 8, 2, 0, 0, 0)
+    ihdr = png_chunk(b"IHDR", ihdr_data)
+    idat = png_chunk(b"IDAT", compressed)
+    iend = png_chunk(b"IEND", b"")
+
+    return b"\x89PNG\r\n\x1a\n" + ihdr + idat + iend
+
+
+BLUE = (37, 99, 235)
+WHITE = (255, 255, 255)
+SIZE = 64
+
+pixels = []
+for y in range(SIZE):
+    row = []
+    for x in range(SIZE):
+        dx = x - SIZE // 2
+        dy = y - SIZE // 2
+        dist = (dx * dx + dy * dy) ** 0.5
+        if dist < SIZE * 0.25:
+            row.append(WHITE)
+        else:
+            row.append(BLUE)
+    pixels.append(row)
+
+with open("icon.png", "wb") as f:
+    f.write(create_png(SIZE, SIZE, pixels))
+
+print("Generated icon.png")