Fix duplicate DOM elements from custom game JS on restart

[alexwarren]

Summary

• "Start from the beginning" / Load reuses the live DOM instead of doing a fresh page load, but previously only ever cleared the output pane. Games commonly use their external <javascript> resources to inject elements elsewhere in the chrome (e.g. a custom sidebar pane), and that setup code re-runs on every restart (RegisterExternalScripts fires on every Initialise call, and a game's StartGame function can call JS.* directly too) — so anything the previous run injected was still there when the new run added another one.
• WasmPlayer: restartGame() now rebuilds the whole player chrome via a new swapInPlayerUi() helper, same as the initial boot, instead of only resetting #divOutput. Also dedupes addExternalScript() by URL in the shared player.js so a script's top-level setup code doesn't re-execute across restarts.
• WebPlayer: mirrors the same fix — Game.razor wraps its rendered chrome in #qv-player-chrome (Blazor never re-diffs it once rendered, since the underlying string is constant) and resets it via WebPlayer.resetPlayerUi() before a restart.

Test plan

• dotnet build --configuration Release (full solution)
• dotnet test --configuration Release (all projects pass)
• Playwright regression test against WasmPlayer dev server (tests/e2e/verify-wasmplayer-restart-sidebar-dup.mjs + fixture game tests/e2e/fixtures/restart-test.aslx): confirms exactly 1 injected pane survives repeated restarts; confirmed the old code duplicates it (1 → 2 → 3) to validate the test is meaningful
• Playwright regression test against a locally-run WebPlayer (tests/e2e/verify-webplayer-restart-chrome-reset.mjs): confirms a manually-injected chrome marker is wiped on restart and gameplay still works afterwards; confirmed the old code leaves the marker in place

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