feat(engine): "Copy share link" Stats button — clipboards the replay URL

Quat: replay sharing as the next punch-list item.

End-to-end:

1. Player wins a game on a server-backed sync backend.
2. `sync_plugin::push_replay_on_win` spawns the upload task on
   `AsyncComputeTaskPool` and stores the handle in the new
   `PendingReplayUpload` resource. The previous in-flight task (if
   any) is dropped — the most recent win is the one whose share link
   the player will care about.
3. `poll_replay_upload_result` harvests the task on the main thread
   each frame; on success writes `<server>/replays/<id>` to
   `LastSharedReplayUrl`. `UnsupportedPlatform` (LocalOnlyProvider)
   is silently absorbed; real network/auth errors warn-log.
4. The Stats overlay's action bar gains a "Copy share link" button.
   Click writes `LastSharedReplayUrl` to the OS clipboard via
   `arboard` and surfaces a "Copied: <url>" toast.

Trait change: `SyncProvider::push_replay` now returns `Result<String,
SyncError>` (the share URL) instead of `Result<(), SyncError>`. The
default (`UnsupportedPlatform`) is unchanged for non-server backends;
`SolitaireServerClient` parses the response body's `id` field and
composes `<base_url>/replays/<id>`. Both call paths (initial + 401
retry) go through the new `share_url_from_response` helper so the
parse logic isn't duplicated.

New deps:
- `arboard` (~10 KB, cross-platform clipboard) added to workspace +
  `solitaire_engine`. `default-features = false` keeps the X11/Wayland
  binary-feature deps off the dependency graph; arboard handles the
  fallback. Approved per the ASK BEFORE rule.

Persistence: the URL is in-memory only — the player must share within
the session of the win. A future revision can persist it alongside
the replay history file if cross-session sharing is needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -30,6 +30,7 @@ dirs        = "6"
 keyring     = "4"
 keyring-core = "1"
 reqwest     = { version = "0.13", features = ["json", "rustls", "rustls-native-certs"], default-features = false }
+arboard     = { version = "3", default-features = false }
 
 solitaire_core   = { path = "solitaire_core" }
 solitaire_sync   = { path = "solitaire_sync" }