Fix #578: a drag on the Color panel's spectrum no longer flickers near white

[bflatastic]

Dragging a colour towards white in the Color panel no longer makes the object flicker.

Cause: the out-of-gamut and out-of-web warnings under the Color panel's proxy appeared only when the colour needed them, so the panel's height changed with the colour. Along the spectrum's pale top edge, a colour out of the CMYK gamut added a warning row. The spectrum then moved 10 px down under the pointer, the pointer was now above it and picked white, the warning went away and the spectrum moved back up. With the pointer held still, the fill alternated every frame between the pale colour and #FFFFFF. A headless probe saw 310 flips in 400 frames. Darker colours don't flicker because they don't toggle the gamut warning.

Fix: panels/color.rs keeps room, as invisible chips, for the warnings the panel's mode can show (gamut in RGB/HSB, web outside Web Safe RGB and Grayscale). The panel's height now depends on the mode, not the colour. When no warning is showing, RGB mode has about 10 px of empty space under the proxy.

• Test a_drag_on_the_spectrum_holds_still_near_white:
  • The panel has one height for white, #fefefe (web warning) and #ded3ff (both warnings).
  • During a drag along the spectrum's top edge, the fill holds while the pointer stops.
  • It fails on main with heights [226.375, 226.375, 236.375].
• ROADMAP Last updated.

Checked live on Windows:

• The Color panel looks the same with #DED3FF (both warnings shown) and #FFFFFF (none); the spectrum stays in place.
• A drag along the spectrum's top ends on the pale colour under the pointer.
• I could not check on macOS, where the issue was reported.

Checks

• Based on origin/main d066e0d.
• cargo test -p vectorcraft-ui-egui, cargo clippy -p vectorcraft-ui-egui --all-targets -- -D warnings, cargo fmt --all, cargo xtask wasm.

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