[alosslessdev]

Fixes #1784.

The Draw tab's pens now show what they draw: Excel-style labels with colour and thickness, and icons painted in the pen's ink colour with a distinct highlighter shape.

What changed

• Labels (crates/ui-egui/src/ribbon.rs): each pen reads Pen: Blue, 2 pt, Pen: Black, 2 pt, Pen: Red, 2 pt, Highlighter: Yellow, 12 pt (Excel's Pen: Black, 0.35 mm shape; GridCraft's unit is points). The active pen shows its current width; pt_label formats whole and half values.
• Icons (icons.rs, widgets.rs): the pen glyph is drawn in the pen's ink colour through a new big_button_tinted (the plain big_button keeps its signature), and a new Icon::Highlighter draws a broad marker with a chisel nib so the highlighter no longer looks like a pen. The highlighter is tinted yellow.

Order, menus and the live preview are unchanged; they are separate PRs (#1783, #1785, #1786). Cut from main, independent of #1767/#1187.

Verification

• cargo fmt --all -- --check
• cargo clippy -p gridcraft-ui-egui --all-targets -- -D warnings
• cargo test -p gridcraft-ui-egui (full crate suite)
• Offscreen render (--example snapshot -- --tab Draw): blue/black/red pen glyphs in their ink colours, a yellow highlighter marker, and the labels with colour and thickness.
• docs/ui-parity.md revision history updated; contributors/people.toml entry per AGENTS.md.

[alosslessdev]

Closing in favour of #1187, which replaces the fixed pen buttons with the editable pen gallery. #1784 is tracked on #1187.