feat: bind cloud plans to provider account scope

[seonghobae]

Summary

• carry provider-authoritative account scope in capacity schema v3
• refine unknown iCloud and OneDrive roots before candidate and review fingerprints are calculated
• reuse one root-bound capacity snapshot across plan, review, and copy paths in the CLI and Tauri app
• fail closed on provider or account-scope conflicts; keep Google Drive scope unknown
• document the account-scope binding contract

Evidence

• cargo check --manifest-path src-tauri/Cargo.toml --all-targets --features cloud-cli
• cargo test --manifest-path src-tauri/Cargo.toml --lib provider_capacity::tests (12 passed)
• cargo test --manifest-path src-tauri/Cargo.toml --features cloud-cli --bin disksage-cloud-plan (15 passed)
• live brctl quota plan: iCloud scope personal, provider-native capacity fit true, review reason reduced to download-origin-needs-destination-review

Safety

Read-only provider metadata only. This PR does not copy, evict, delete, or approve any user file.

[coderabbitai]

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