feat: add Cubence sponsor preset

[SaladDay]

Summary

• replace RightCode with Cubence in English and Chinese README sponsor sections
• add Cubence sponsor preset across Claude, Codex, Gemini, OpenCode, Hermes, and OpenClaw
• add focused provider form tests for Cubence URLs and template ordering

Tests

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo test --manifest-path src-tauri/Cargo.toml provider_add_form_template_labels_follow_explicit_support_matrix
• cargo test --manifest-path src-tauri/Cargo.toml provider_add_form_cubence