Part of the Godot→Flutter migration (#79).

Current state

iOS has no update path and no release artifact. platformAssetUrl() returns null for iOS with the comment "iOS updates come from the App Store" (update_controller.dart:202-203), but there is no App Store / TestFlight presence, and release.yml produces no iOS build. So iOS users currently can neither install nor update through any supported channel.

Decision needed

Pick the iOS distribution model, then implement it:

• App Store — real distribution; requires Apple Developer account, signing, review. Updates handled by the OS/store.
• TestFlight — lower friction for early access; 90-day build expiry; still needs an Apple account.
• Defer/drop iOS for v1 — explicitly hide iOS from the update UI and document it as unsupported for now.

Follow-on work (if shipping iOS)

• Add an iOS build + signing job to release.yml.
• Wire the update UI to deep-link to the App Store/TestFlight instead of returning null.

Acceptance

A documented, working answer for "how does an iOS user get and update the app", reflected in both CI and the in-app update UI.

Migrated from DaccordProject/daccord-app#82