feat: Cargo target GC, relay BuildKit cache, and sync quota relief

[bobleer]

Summary

• Prune stale Cargo target caches (incremental / fingerprint / deps) after desktop:dev, preview shutdown, and desktop:build*, keeping only the latest useful artifacts; add pnpm run target:gc.
• Speed relay redeploys with Docker BuildKit Cargo registry/git/target cache mounts, pin the builder toolchain, and propagate DOCKER_BUILDKIT=1 through deploy / one-click paths.
• Relieve cloud session backup pressure: server-side LRU eviction on upsert, client-side 507 quota relief with clearer sync errors (no blind retries for 507/413), and raise default per-user sync caps for BitFun deployments.
• Temporarily hide the Pages nav entry; trim relay Tokio features and simplify the account security note copy.

Test plan

• node --test scripts/cargo-target-gc.test.mjs
• BITFUN_TARGET_GC_DRY_RUN=1 pnpm run target:gc -- --profile debug against a local target/debug and confirm keep-latest counts
• Exit pnpm run desktop:dev and confirm target GC logs without failing the session
• cargo test -p bitfun-relay-service sync_session_make_room_evicts_oldest_until_upsert_fits -- --nocapture
• cargo test -p bitfun-services-integrations detects_insufficient_storage_errors_for_quota_relief -- --nocapture (or nearest focused lib test filter)
• Redeploy relay with BuildKit enabled and confirm cache mounts reuse across a second build
• Account sync: force a full quota path and confirm oldest cloud backups are evicted / UI shows quota or payload-too-large copy instead of generic retry
• Confirm Pages entry is hidden in MainNav; MiniApp entry still works
• pnpm run i18n:audit for locale key additions
• pnpm run type-check:web for AccountPanel / MainNav changes