feat(config): support launch viewport defaults

[EfeDurmaz16]

Summary

• add viewport and deviceScaleFactor config keys for persistent launch viewport defaults
• apply the configured viewport during launch/reuse so screenshots can stay deterministic across daemon restarts
• update schema, README, docs, CLI help, and core skill references

Fixes #1356.

Testing

• cargo fmt --manifest-path cli/Cargo.toml -- --check
• cargo test --manifest-path cli/Cargo.toml flags::tests::
• cargo test --manifest-path cli/Cargo.toml commands::tests::test_set_viewport
• cargo test --manifest-path cli/Cargo.toml --no-run
• cargo clippy --manifest-path cli/Cargo.toml -- -D warnings
• git diff --check

[vercel]

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