fix(selection): a fast press-drag starts the selection window at once

A left drag only became a lasso/window after the button had been held
150 ms, however far the cursor had moved. A fast flick released inside
that delay read as a click and armed a click-click box instead. A press
that has moved more than 12 px is now a drag at once; the delay still
keeps small movements of a click from becoming one. (#1808)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/mod.rs

@@ -378,6 +378,9 @@ use iced::{mouse, Point, Task, Theme};
 use std::sync::Arc;
 
 pub(super) const POLY_START_DELAY_MS: u128 = 150;
+/// Squared screen distance (12 px) past which a left press is a drag
+/// without waiting for `POLY_START_DELAY_MS`.
+pub(super) const DRAG_START_FAST_PX2: f32 = 144.0;
 pub(super) const VARIES_LABEL: &str = "*VARIES*";
 
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