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/button/mod.rs

@@ -79,6 +79,11 @@ pub struct Button<Msg: Clone>
 	/// surface so it does not stay frozen while the rest of a fluid layout
 	/// grows. Resolved in physical layout space, like all geometry.
 	pub( crate ) height:    Option<Length>,
+	/// Optional fixed width for text buttons. `None` sizes the button to its
+	/// label plus horizontal padding; a [`Length`] pins the width (e.g. to
+	/// make a full-width or surface-proportional button), clamped to the
+	/// available width. Resolved in physical layout space.
+	pub( crate ) width:     Option<Length>,
 	/// Optional stable identifier for focus management.
 	pub( crate ) id:        Option<WidgetId>,
 	/// Whether this button participates in keyboard focus (Tab). Default: `true`.
@@ -113,6 +118,7 @@ impl<Msg: Clone> Button<Msg>
 			icon_size:     0.0,
 			font_size:     None,
 			height:        None,
+			width:         None,
 			id:            None,
 			focusable:     true,
 			cursor:        None,
@@ -150,6 +156,7 @@ impl<Msg: Clone> Button<Msg>
 			icon_size:     0.0,
 			font_size:     None,
 			height:        None,
+			width:         None,
 			id:            None,
 			focusable:     true,
 			cursor:        None,
@@ -256,6 +263,16 @@ impl<Msg: Clone> Button<Msg>
 		self
 	}
 
+	/// Pin the button width for text buttons. Accepts logical `f32` pixels or
+	/// any [`Length`] (e.g. `Length::orient( 95.0, 25.0 )` for a
+	/// surface-proportional width). Without this the button sizes to its
+	/// label. Clamped to the available width. No-op for icon buttons.
+	pub fn width( mut self, w: impl Into<Length> ) -> Self
+	{
+		self.width = Some( w.into() );
+		self
+	}
+
 	/// Resolve the label font size against the canvas viewport, matching how
 	/// [`text`](crate::text) sizes its glyphs. An explicit override bypasses
 	/// the mode; the default follows the process [`crate::WidgetScaling`].
@@ -329,8 +346,15 @@ impl<Msg: Clone> Button<Msg>
 		{
 			ButtonContent::Text( label ) =>
 			{
-				let text_w = canvas.measure_text( label, self.label_font_size( canvas ) );
-				let w = (text_w + canvas.geom_px( theme::PAD_H ) * 2.0).min( max_width );
+				let w = match self.width
+				{
+					Some( l ) => l.resolve( canvas.viewport_layout(), Length::EM_BASE_DEFAULT ).min( max_width ),
+					None      =>
+					{
+						let text_w = canvas.measure_text( label, self.label_font_size( canvas ) );
+						( text_w + canvas.geom_px( theme::PAD_H ) * 2.0 ).min( max_width )
+					}
+				};
 				( w, self.resolved_height( canvas ) )
 			}
 			ButtonContent::Icon { .. } =>
@@ -510,6 +534,7 @@ impl<Msg: Clone> Button<Msg>
 			icon_size:     self.icon_size,
 			font_size:     self.font_size,
 			height:        self.height,
+			width:         self.width,
 			id:            self.id,
 			focusable:     self.focusable,
 			cursor:        self.cursor,