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