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.lock

@@ -44,7 +44,7 @@ dependencies = [
  "accesskit_consumer",
  "hashbrown 0.15.5",
  "objc2 0.5.2",
- "objc2-app-kit",
+ "objc2-app-kit 0.2.2",
  "objc2-foundation 0.2.2",
 ]
 
@@ -313,6 +313,23 @@ dependencies = [
  "num-traits",
 ]
 
+[[package]]
+name = "arboard"
+version = "3.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0348a1c054491f4bfe6ab86a7b6ab1e44e45d899005de92f58b3df180b36ddaf"
+dependencies = [
+ "clipboard-win",
+ "log",
+ "objc2 0.6.4",
+ "objc2-app-kit 0.3.2",
+ "objc2-foundation 0.3.2",
+ "parking_lot",
+ "percent-encoding",
+ "windows-sys 0.60.2",
+ "x11rb",
+]
+
 [[package]]
 name = "arc-swap"
 version = "1.9.1"
@@ -1972,6 +1989,15 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c8d4a3bb8b1e0c1050499d1815f5ab16d04f0959b233085fb31653fbfc9d98f9"
 
+[[package]]
+name = "clipboard-win"
+version = "5.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bde03770d3df201d4fb868f2c9c59e66a3e4e2bd06692a0fe701e7103c7e84d4"
+dependencies = [
+ "error-code",
+]
+
 [[package]]
 name = "cmake"
 version = "0.1.58"
@@ -2882,6 +2908,12 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "error-code"
+version = "3.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dea2df4cf52843e0452895c455a1a2cfbb842a1e7329671acf418fdc53ed4c59"
+
 [[package]]
 name = "etcetera"
 version = "0.8.0"
@@ -4968,6 +5000,18 @@ dependencies = [
  "objc2-quartz-core",
 ]
 
+[[package]]
+name = "objc2-app-kit"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d49e936b501e5c5bf01fda3a9452ff86dc3ea98ad5f283e1455153142d97518c"
+dependencies = [
+ "bitflags 2.11.1",
+ "objc2 0.6.4",
+ "objc2-core-graphics",
+ "objc2-foundation 0.3.2",
+]
+
 [[package]]
 name = "objc2-audio-toolbox"
 version = "0.3.2"
@@ -5065,6 +5109,19 @@ dependencies = [
  "objc2 0.6.4",
 ]
 
+[[package]]
+name = "objc2-core-graphics"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e022c9d066895efa1345f8e33e584b9f958da2fd4cd116792e15e07e4720a807"
+dependencies = [
+ "bitflags 2.11.1",
+ "dispatch2",
+ "objc2 0.6.4",
+ "objc2-core-foundation",
+ "objc2-io-surface",
+]
+
 [[package]]
 name = "objc2-core-image"
 version = "0.2.2"
@@ -5121,6 +5178,17 @@ dependencies = [
  "objc2-core-foundation",
 ]
 
+[[package]]
+name = "objc2-io-surface"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "180788110936d59bab6bd83b6060ffdfffb3b922ba1396b312ae795e1de9d81d"
+dependencies = [
+ "bitflags 2.11.1",
+ "objc2 0.6.4",
+ "objc2-core-foundation",
+]
+
 [[package]]
 name = "objc2-link-presentation"
 version = "0.2.2"
@@ -5129,7 +5197,7 @@ checksum = "a1a1ae721c5e35be65f01a03b6d2ac13a54cb4fa70d8a5da293d7b0020261398"
 dependencies = [
  "block2 0.5.1",
  "objc2 0.5.2",
- "objc2-app-kit",
+ "objc2-app-kit 0.2.2",
  "objc2-foundation 0.2.2",
 ]
 
@@ -6856,6 +6924,7 @@ dependencies = [
 name = "solitaire_engine"
 version = "0.1.0"
 dependencies = [
+ "arboard",
  "async-trait",
  "bevy",
  "chrono",
@@ -9516,7 +9585,7 @@ dependencies = [
  "libc",
  "ndk",
  "objc2 0.5.2",
- "objc2-app-kit",
+ "objc2-app-kit 0.2.2",
  "objc2-foundation 0.2.2",
  "objc2-ui-kit",
  "orbclient",