fix(balance): add Decred support

[oritwoen]

Summary

This finishes the remaining Decred half of #117.

• add Decred balance fetching via dcrdata totals endpoint
• map dcrdata invalid-address responses to boha's existing InvalidAddress error
• cover the new path with ignored live tests for a known address and an invalid address

Verification

• cargo test --all-features dcr -- --ignored
• cargo test --all-features
• cargo fmt --check
• cargo clippy --all-features -- -D warnings
• cubic review
• coderabbit review (interactive TUI launched, no non-interactive verdict available here)

Closes #117.

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: Adding support for a new blockchain (Decred) involves external API integration and business logic changes that require human validation.}

Architecture diagram

sequenceDiagram
    participant C as Caller
    participant B as balance::fetch
    participant D as dcrdata API (External)

    C->>B: fetch(address, Chain::Decred)
    
    B->>D: NEW: GET /api/address/{address}/totals
    
    alt Success (200 OK)
        D-->>B: { dcr_unspent: f64 }
        B->>B: NEW: Convert DCR to atomic units (f64 * 10^8)
        B-->>C: Ok(Balance)
    else Invalid Address (400 or 422)
        D-->>B: Error Status
        Note over B: CHANGED: is_invalid_address_status() helper
        B-->>C: Err(BalanceError::InvalidAddress)
    else Connection/Server Error
        D-->>B: Request Failure
        B-->>C: Err(BalanceError::Request)
    end

    Note over B,D: Also updates fetch_mempool_compatible to use 422 check

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