Fix scan-to-download auth and share every photo widget's current photo

The share QR's URL carried no auth params at all, so it silently fell
back through require_device's legacy-token resolution to whichever
frame happened to still be flagged legacy -- working only by accident
for a single frame, sharing the wrong frame's photos for any other, and
going fully dead once that frame's legacy flag was cleared.

Move the endpoint to manage.py, keyed on the frame's own manage_token
(same pattern /m/<manage_token> already uses) instead of device auth.
Since the server now resolves assets itself instead of trusting a
caller-supplied asset_id, it naturally generalizes to gather every
photo widget's current photo into one Immich share link, not just one
"primary" widget's.

server/app/immich_client.py

@@ -80,10 +80,10 @@ class ImmichClient:
         resp.raise_for_status()
         return resp.json()
 
-    def create_share_link(self, asset_id: str, expires_in_s: int) -> str:
-        """Creates a public, view-only Immich share link for a single
-        asset, expiring expires_in_s seconds from now, and returns its
-        public URL. Used by the manage-button overlay's share QR --
+    def create_share_link(self, asset_ids: list[str], expires_in_s: int) -> str:
+        """Creates a public, view-only Immich share link covering one or
+        more assets, expiring expires_in_s seconds from now, and returns
+        its public URL. Used by the manage-button overlay's share QR --
         created lazily (only when someone actually scans it), not when
         the button's pressed, so the expiry clock starts when it's
         actually used."""
@@ -93,7 +93,7 @@ class ImmichClient:
             headers=self._headers,
             json={
                 "type": "INDIVIDUAL",
-                "assetIds": [asset_id],
+                "assetIds": list(asset_ids),
                 "expiresAt": expires_at,
                 "allowUpload": False,
                 "allowDownload": True,