feat(profile): derive the profile P&L from the real round ledger

[netqo]

Summary

Phase 2 (final) of the stats layer: the profile's P&L and game-activity blocks now read the persisted game_round ledger instead of placeholder numbers. With history and round detail already wired, this closes the stats layer - the three round-consuming screens all show real data.

Rationale

ProfileViewModel combines AuthRepository.currentUser (the real identity, already wired) with GameRepository.rounds (the real rounds). The identity, Error and sign-out behaviour are unchanged; only the P&L and game-activity blocks move from placeholders to computed values:

• All-time P&L: net (= won - wagered), bets, win rate, won and wagered, plus the won/wagered proportion for the split bar.
• Per-game activity: rounds grouped by game (bets, wagered, net per game), ordered by volume, each with its by-volume bar.
• Empty ledger: zeros plus a "No game activity yet" empty state in the activity card, so a fresh account shows nothing invented.

Kept scoped to the ViewModel + the activity empty state; the screen composables and the data shapes (ProfilePnl, GameActivityRow) are unchanged.

Verification

• ./gradlew testDebugUnitTest detekt ktlintCheck assembleDebug compileDebugAndroidTestKotlin green locally (JDK 17).
• ProfileViewModelTest extended: identity/error/sign-out cases preserved, plus new cases asserting the P&L and game-activity derivation from rounds.
• Installed the debug APK on a physical device: played coinflip rounds, confirmed the P&L summary and the Coinflip activity row reflect the real wagered/net, and that a fresh account shows zeros + "No game activity yet".

Test plan

• P&L summary (net, bets, win rate, won/wagered) reflects the played rounds.
• Game Activity lists one row per game played with correct bets/wagered/net.
• An empty ledger shows zeros and "No game activity yet".
• Identity, settings sheet and Sign Out still work.

Related issues

Refs the stats-layer work for the final entrega (phase 2 of 2; closes the stats layer).

Checklist

• Commit messages follow Conventional Commits (see CONTRIBUTING.md).
• Branch name follows the naming convention (feat/, fix/, chore/, etc.).
• The change is scoped: no unrelated edits sneaked into this PR.
• If user-visible behavior changed, CHANGELOG.md has been updated under [Unreleased].
• If a new dependency was added, it is justified in the PR description. (No new dependencies.)