fix(ui): robustly center the Settings header title

The sticky Settings header centered its title with two horizontal_space()
flanks plus a hardcoded 90px right spacer guessing the Back button's width —
off-center if the button's rendered width drifted (documented follow-up).
Replace with equal-width Fill flanks: the Back button sits in a left Fill
segment (left-aligned), an empty Fill segment balances the right, so the
center title is geometrically centered regardless of button width. No magic
constant. Pure layout change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1325,27 +1325,29 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
         .height(iced::Length::Fill);
 
         // Sticky header bar: stays fixed above the scrollable content so the Back
-        // button is always reachable. The "Settings" title is centered, with a
+        // button is always reachable. The "Settings" title is centered by flanking
-        // right-side spacer matching the Back button's width to keep it visually
+        // it with two equal-width Fill segments — the Back button lives in the left
-        // centered.
+        // one (left-aligned) and the right one is an empty balance, so the title is
+        // mathematically centered regardless of the Back button's rendered width.
         let settings_header = container(
             row![
-                button(
+                container(
-                    row![
+                    button(
-                        text("←").size(16),
+                        row![
-                        text("Back").size(14),
+                            text("←").size(16),
-                    ]
+                            text("Back").size(14),
-                    .spacing(6)
+                        ]
-                    .align_y(iced::alignment::Vertical::Center)
+                        .spacing(6)
+                        .align_y(iced::alignment::Vertical::Center)
+                    )
+                    .on_press(AppMessage::NavigateBack)
+                    .style(b_style(color_surface, color_blue, color_text, 8.0))
+                    .padding([8, 14])
                 )
-                .on_press(AppMessage::NavigateBack)
+                .width(iced::Length::Fill),
-                .style(b_style(color_surface, color_blue, color_text, 8.0))
-                .padding([8, 14]),
-                horizontal_space(),
                 text("Settings").size(20).color(color_blue),
-                horizontal_space(),
+                // Equal-width balance spacer keeps the title centered.
-                // Balance spacer so the title stays centered against the Back button.
+                container(text("")).width(iced::Length::Fill),
-                container(text("")).width(iced::Length::Fixed(90.0)),
             ]
             .align_y(iced::alignment::Vertical::Center)
             .width(iced::Length::Fill),