Show the OIDC profile picture with an identicon fallback

GET /api/me now carries the profile picture URL when the provider
supplies an absolute http(s) one; the SPA shows it in the lobby navbar
and draws a deterministic identicon from the sub otherwise, falling back
to the identicon when the image fails to load.

The identicon is a 5x5 mirrored grid seeded by an inline FNV-1a hash so
it stays stable across Rust versions and is unit-testable on the host.
The dev mock IdP hands alice and bob a picture claim.

server/dev/mockoauth/config.json

@@ -11,7 +11,8 @@
           "claims": {
             "sub": "alice",
             "preferred_username": "alice",
-            "name": "Alice"
+            "name": "Alice",
+            "picture": "https://i.pravatar.cc/128?u=alice"
           }
         },
         {
@@ -20,7 +21,8 @@
           "claims": {
             "sub": "bob",
             "preferred_username": "bob",
-            "name": "Bob"
+            "name": "Bob",
+            "picture": "https://i.pravatar.cc/128?u=bob"
           }
         },
         {