feat(ui): add style module — deep-space cyan/orange theme

Palette constants + sci_fi_visuals() + setup(). Hand-applied over
Visuals::dark(); no custom font, no new deps. Not yet wired into
ui_system (Task 4 does that via setup_egui_style).

src/ui/mod.rs

@@ -1,3 +1,5 @@
+mod style;
+
 use bevy::prelude::*;
 use bevy_egui::egui;
 

src/ui/style.rs

@@ -0,0 +1,80 @@
+//! egui styling for the control panel. Deep-space cyan/orange theme,
+//! hand-applied over `Visuals::dark()`. See design spec §1.
+
+use bevy_egui::egui::{self, Color32, Stroke, TextStyle, Visuals};
+
+// --- Palette (single source of truth; referenced by every section helper) ---
+pub const ACCENT_CYAN: Color32 = Color32::from_rgb(90, 200, 255);   // #5AC8FF
+pub const ACCENT_ORANGE: Color32 = Color32::from_rgb(255, 140, 66); // #FF8C42
+pub const PANEL_FILL: Color32 = Color32::from_rgb(14, 16, 20);      // #0E1014
+pub const EXTREME_BG: Color32 = Color32::from_rgb(8, 9, 12);        // #08090C
+pub const MUTED_TEXT: Color32 = Color32::from_rgb(140, 140, 140);   // read-only / disabled labels
+pub const DIM_TEXT: Color32 = Color32::from_rgb(110, 110, 110);     // section-disabled headers
+
+/// Text sizes applied over egui's default `Proportional` family. No custom
+/// font binary is embedded (spec non-goal).
+pub fn text_styles() -> Vec<(TextStyle, f32)> {
+    vec![
+        (TextStyle::Body, 14.0),
+        (TextStyle::Monospace, 12.0),
+        (TextStyle::Heading, 16.0),
+        (TextStyle::Small, 11.0),
+    ]
+}
+
+/// The themed `Visuals`. Built fresh from `dark()` each call so it is
+/// independent of whatever egui's defaults evolve into.
+///
+/// Note: `animation_time` lives on `Style`, not `Visuals`, since egui 0.35;
+/// it is applied in [`setup`].
+pub fn sci_fi_visuals() -> Visuals {
+    let mut v = Visuals::dark();
+    v.panel_fill = PANEL_FILL;
+    v.extreme_bg_color = EXTREME_BG;
+    v.hyperlink_color = ACCENT_CYAN; // also used as the section-heading color by convention
+    v.selection.bg_fill = ACCENT_CYAN;
+    v.selection.stroke = Stroke::new(1.0, ACCENT_ORANGE);
+
+    v.widgets.inactive.weak_bg_fill = Color32::from_rgb(30, 36, 48);
+    v.widgets.inactive.bg_stroke = Stroke::new(0.5, Color32::from_rgb(60, 70, 90));
+    v.widgets.hovered.weak_bg_fill = Color32::from_rgb(40, 50, 70);
+    v.widgets.hovered.fg_stroke = Stroke::new(1.0, Color32::from_rgb(200, 220, 255));
+    v.widgets.active.fg_stroke = Stroke::new(1.0, ACCENT_CYAN);
+    v.widgets.active.weak_bg_fill = Color32::from_rgb(50, 70, 100);
+    v.widgets.noninteractive.bg_stroke = Stroke::new(0.5, Color32::from_rgb(40, 46, 60));
+
+    // egui 0.35's `epaint::Shadow` packs its fields into 8-bit integers
+    // (`offset: [i8; 2]`, `blur: u8`, `spread: u8`) to keep the struct at 8
+    // bytes. Use integer literals; the f32 form would not type-check.
+    v.window_shadow = egui::epaint::Shadow {
+        offset: [0, 4],
+        blur: 16,
+        spread: 0,
+        color: Color32::from_black_alpha(120),
+    };
+    v
+}
+
+/// Apply fonts + spacing + visuals to a context. Called exactly once from
+/// `setup_egui_style` (plugin.rs).
+///
+/// egui 0.35 replaced `ctx.style()` / `ctx.set_style()` with theme-scoped
+/// accessors. We pin the active theme to Dark and mutate the active `Style`
+/// in place; `Visuals` and `animation_time` are both fields of `Style`, so
+/// they go through the same closure. `text_styles` is now a public
+/// `BTreeMap` field (no `text_styles_mut()`).
+pub fn setup(ctx: &egui::Context) {
+    ctx.set_theme(egui::Theme::Dark);
+    ctx.global_style_mut(|style| {
+        style.spacing.item_spacing = egui::vec2(8.0, 6.0);
+        style.spacing.slider_width = 150.0;
+        style.spacing.indent = 18.0;
+        style.spacing.button_padding = egui::vec2(10.0, 4.0);
+        style.spacing.indent_ends_with_horizontal_line = false;
+        style.animation_time = 0.12;
+        for (ts, size) in text_styles() {
+            style.text_styles.insert(ts, egui::FontId::proportional(size));
+        }
+        style.visuals = sci_fi_visuals();
+    });
+}