Make streak protection opt-out by default

[AldanaBRZ]

Streak protection now defaults to enabled when a player has no saved choice. Players can opt out through the existing settings toggle; the first opt-out is recorded even before they upload reviews, repeated saves remain idempotent, and recorded opt-outs survive restarts.

Use one shared preference policy for settings responses, profiles, quest rewards and automatic protection. Persist the rollout time once so upgrading an opt-in server does not retroactively earn or spend freezes or change past streak/XP history. Existing saved freezes and preference timelines remain intact.

Update the dashboard's English/Spanish explanation and README to describe the new default and upgrade behavior. Android's existing toggle reads the server response; AnkiQuest-Android#38 updates the displayed explanation.

Validation:

• Confirmed three new regressions failed before the fix.
• All 231 Rust tests passed on the current server base, including earning without a settings change, first opt-out persistence, upgrade boundaries and saved opt-outs.
• All 21 freeze-dialog browser checks and 6 translation checks passed, including opting out of a checked toggle without losing stock.
• Formatting and Clippy checks passed.

Note

Assisted-by: GPT-6 (implementation, regression tests, review and PR description).