text_edit font sizing on Length; add Button::width and Text::line_height

Unify TextEdit's font size with the button label. The button resolved its `font_size` Length in font space (`viewport_logical`, so the `× dpi_scale` at raster lands the right physical size), while `TextEdit::font_size` took a raw `f32` that the draw / hit-test paths treated as a logical size. A caller that resolved a Length against the physical surface and passed the result as that `f32` therefore double-counted `dpi_scale` and got a font that rendered too large. `TextEdit::font_size` now takes `impl Into<Length>` and is resolved against `viewport_logical` exactly like the button, so the two paths agree. The field stores `Option<Length>` (`None` follows the widget-scaling mode at the theme default), retiring the old `f32` sentinel (`0.0` = mode, negative = fluid design px). `font_size_fluid` becomes shorthand for `Length::fluid( n )`. The `Option<Length>` flows through the `WidgetHandlers::TextEdit` snapshot and `text_input_geometry` and is resolved at draw / hit-test time, both of which carry a canvas; the inner measure helpers (`wrapping`, `hit_test`) keep `f32` because they receive the already-resolved value. Backward compatible: `f32` call sites still compile via `f32: Into<Length>` (→ `Length::px`), with the same result as before.
Add `Button::width( impl Into<Length> )`. Text buttons size to their label plus padding; some layouts need a pinned width instead — a full-width or surface-proportional button. The new builder mirrors `height`: resolved in physical layout space, clamped to the available `max_width`, propagated through `map_msg`, and a no-op for icon buttons.
Add `Text::line_height( mult )`. Wrapped multi-line text used the font's declared leading (`new_line_size`), which is tight for some labels; the multiplier scales the gap between wrapped lines (`1.0`, the default, keeps the natural leading — every other `text` is unchanged — and a `0.5` floor keeps lines from overlapping). Applied uniformly in `preferred_size` and `draw` so the reported height and the drawn baselines stay consistent.
Tests: `button` gains a pinned-width and a size-to-content case; `text` gains a line-height default / clamp test and a check that doubling the line height doubles a wrapped block's reported height. Docs: `docs/widgets.md` `text` / `button` / `separator` sections updated for the new builders, and a `CHANGELOG.md` "Unreleased" section covering this batch alongside the responsive work already landed.

src/widget/handlers.rs

@@ -2,7 +2,7 @@
 // Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>
 
 use std::sync::Arc;
-use crate::types::{ Point, Rect };
+use crate::types::{ Length, Point, Rect };
 use super::{ slider, text };
 
 /// Per-leaf interaction snapshot captured during layout. One variant per
@@ -56,9 +56,9 @@ pub enum WidgetHandlers<Msg: Clone>
 		align:     text::TextAlign,
 		/// Font size snapshot — needed by the hit-testing path so
 		/// the runtime measures glyphs at the same size the renderer
-		/// drew them. Always the default `theme::FONT_SIZE` for
-		/// fields that do not call `.font_size( … )`.
-		font_size: f32,
+		/// drew them. `None` for fields that do not call
+		/// `.font_size( … )` (they follow the widget-scaling mode).
+		font_size: Option<Length>,
 		/// `true` when the source field opted into select-all-on-
 		/// focus. The runtime reads this in `set_focus` to decide
 		/// whether the new selection should anchor at `0` (replace