feat: support creating challenge links

[tom-anders]

Closes #2015
Closes #856

Demo:

challengelink.webm

[Copilot]

Pull request overview

Adds “challenge a friend” support by enabling creation of open challenges (no destination user) and surfacing a shareable link + QR code while waiting for an opponent, addressing the “game invite link” feature requests.

Changes:

• Allow ChallengeRequest.destUser to be nullable and implement /api/challenge/open creation with immediate self-accept (color selection).
• Introduce new game-screen states and loading UI for open challenges, including link/QR sharing.
• Add qr_flutter dependency and adjust call sites to the updated CreateChallengeBottomSheet constructor.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
test/model/challenge/challenge_repository_test.dart	Adds coverage for creating an open challenge and auto-accepting with chosen color.
pubspec.yaml	Adds qr_flutter dependency.
pubspec.lock	Locks qr_flutter and transitive qr.
lib/src/widgets/board.dart	Changes overlay layout to be unconstrained/centered.
lib/src/view/user/user_screen.dart	Updates CreateChallengeBottomSheet call to named param.
lib/src/view/user/online_bots_screen.dart	Updates CreateChallengeBottomSheet call to named param.
lib/src/view/play/play_menu.dart	Adds “Challenge a friend” entry that opens the challenge bottom sheet with user: null.
lib/src/view/play/create_challenge_bottom_sheet.dart	Makes user nullable via named constructor args.
lib/src/view/game/game_screen_providers.dart	Adds open/user challenge “created” states and splits challenge creation vs response waiting.
lib/src/view/game/game_screen.dart	Adds rendering for OpenChallengeCreatedState and renames loading widget for user challenges.
lib/src/view/game/game_loading_board.dart	Adds open-challenge waiting UI with link copy + QR + share button; refactors overlay card UI.
lib/src/view/board_editor/board_editor_screen.dart	Updates CreateChallengeBottomSheet call to named params.
lib/src/model/lobby/create_game_service.dart	Splits challenge creation from waiting for challenge response.
lib/src/model/challenge/challenge_repository.dart	Implements /api/challenge/open create + immediate accept; adds accept color query param.
lib/src/model/challenge/challenge_preferences.dart	Allows makeRequest to accept nullable destination user.
lib/src/model/challenge/challenge.dart	Makes ChallengeRequest.destUser nullable and documents open-challenge semantics.

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[veloce]

I'll put this on hold until we figure out how to deal with anon challenge creation.

[tom-anders]

@veloce Fix for anon challenges is here: lichess-org/lila#20101 so I think you can start reviewing already if you have the time

[veloce]

Code looks good!

Now I need some time to test this properly.

[veloce]

Perhaps find a better icon here. I'll think about it. Like a link or qr code icon maybe.

[tom-anders]

Hmm, on the website it uses the friend icon.

Btw, as a follow-up, I’m planning to add a „or challenge a friend“ button to the „share challenge link“ dialog which would open the friends screen and allow you to pick a user, turning the open challenge into a directed one. The website has the same functionality, what do you think about adding it here as well?