Redesign Select Color panel: index and true color tabs

Replace the old index page and the iced_aw ColorPicker popup with one
Select Color panel:

- Index Color tab: standard colors, ACI 10-249 grid, grays; arrow keys
  move through the palette.
- True Color tab: HSB, RGB and gradient-field modes built on two new
  widgets (GradientSlider, ColorArea) and iced_aw NumberInput boxes,
  an editable hex field and a nearest/exact index color button.
- Current/New swatch: clicking Current reverts. A mixed selection opens
  on the color of the most recently drawn object with *VARIES* on top.
- Color field accepts an index, a color name, r,g,b or hex.
- Enter confirms, Esc cancels, other keys stay in the panel.
- New labels translated in all 22 locales.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/locale_catalog.rs

@@ -4694,6 +4694,9 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Index Color" => Some(("common", "index-color")),
         "True Color" => Some(("common", "true-color")),
         "Standard colors" => Some(("common", "standard-colors")),
+        "Hex" => Some(("common", "color-hex")),
+        "Nearest index" => Some(("common", "nearest-index-color")),
+        "Exact index match" => Some(("common", "exact-index-match")),
         "CAD Color Index (ACI) 10–249" => Some(("common", "cad-color-index-aci-10-249")),
         "Color family  →    ·    Shade / intensity  ↓" => Some(("common", "color-family-shade-intensity")),
         "Grayscale 250–255" => Some(("common", "grayscale-250-255")),