feat: add picker action to placeholder page

[floze-the-genius]

Summary

• restore a prominent Pick a Color action on the empty-state page
• reuse the existing win.pick-color action used by the header button
• keep manual color entry available while making its View button secondary

Validation

• blueprint-compiler compile --output /tmp/placeholder.ui data/resources/ui/placeholder-page.blp
• xmllint --noout /tmp/placeholder.ui
• ninja -C <build-dir> data/resources/resources.gresource
• cargo fmt --all -- --check
• git diff --check

A full Meson build generated the Blueprint/resources successfully, then stopped in the Rust dependency build on macOS because the Linux-only gtk4-wayland and gtk4-x11 pkg-config files are unavailable locally. The repository CI remains authoritative for the Linux build.

Closes #234

[FineFindus]

Thanks for your interest in contributing to this project. I don't think simply adding a second button is an elegant solution.

A full Meson build generated the Blueprint/resources successfully, then stopped in the Rust dependency build on macOS because the Linux-only gtk4-wayland and gtk4-x11 pkg-config files are unavailable locally. The repository CI remains authoritative for the Linux build.

This is not enough, as many GTK things are only checked at runtime, not at build time.

Additionally, as the pull request description unfortunately looks like it's AI-generated, I must assume that the same applies for the code submitted here, and hence I must reject this PR (especially since the issues is intentionally left open for a Human contributor to learn about the project).
I don't allow AI contributions because the copyright situation of such code is unclear, and I believe that the extensive use of AI code might cause maintainability and code quality issues in the future.
You're welcome to contribute using non-AI code.

Thanks for understanding.