Refactor: extract shared image upload utilities into bincio/shared/images.py

ALLOWED_IMAGE_TYPES, MAX_IMAGE_BYTES, and unique_image_name() were
duplicated identically in both the edit and serve servers. Centralising
them means a single change point for any future extension (e.g. adding
image/avif support).

Tests added in tests/test_shared_images.py cover no-collision, single
and chained collisions, no-suffix filenames, and constant values.

bincio/serve/server.py

@@ -332,21 +332,9 @@ def _set_session_cookie(response: Response, token: str) -> None:
     response.set_cookie(**kwargs)
 
 
-# ── Image upload constants ────────────────────────────────────────────────────
-
-_ALLOWED_IMAGE_TYPES = {"image/jpeg", "image/png", "image/webp", "image/gif"}
-_MAX_IMAGE_BYTES = 10 * 1024 * 1024  # 10 MB
-
-
-def _unique_image_name(directory: Path, filename: str) -> str:
-    """Return a filename that does not collide with existing files in directory."""
-    stem, suffix = Path(filename).stem, Path(filename).suffix
-    candidate = filename
-    counter = 1
-    while (directory / candidate).exists():
-        candidate = f"{stem}_{counter}{suffix}"
-        counter += 1
-    return candidate
+from bincio.shared.images import ALLOWED_IMAGE_TYPES as _ALLOWED_IMAGE_TYPES
+from bincio.shared.images import MAX_IMAGE_BYTES as _MAX_IMAGE_BYTES
+from bincio.shared.images import unique_image_name as _unique_image_name
 
 
 # ── Post-write rebuild ────────────────────────────────────────────────────────