Multi-account Phase 3 PR C: UI wiring in MenuContentView + SettingsContentView

[hanrw]

Context

Phase 3 follow-up from #160 — Concrete PR C (self-contained, independently mergeable).

Scope

• AccountPickerView in providerSection(provider:) — conditional on provider is MultiAccountProvider && accounts.count > 1
• AccountManagementCard in SettingsContentView after provider-specific config cards — same conformance check
• accountCard shows active account info for multi-account providers
• Minimal diff — mostly if let multi = provider as? MultiAccountProvider guards

Dependencies

Depends on PR A (#164) and PR B for full functionality.

Ref: #160 (Phase 3 discussion in PR comments)