feat: implement evolution and gradient systems with settings modal

- Add EvolutionController to manage spawning and physics of icons.
- Introduce GradientController for animated gradient backgrounds.
- Create iconLoader for fetching and caching SVG icons.
- Implement modal functionality for settings adjustments.
- Connect UI controls to controllers via SettingsController.
- Add CSS styles for gradient animations, icons, and modal.
- Create utility functions for color manipulation in gradients.

src/js/constants.js

@@ -0,0 +1,43 @@
+/**
+ * constants.js
+ * Single source of truth for all application defaults and tuneable values.
+ * Import from here — never hardcode magic numbers elsewhere.
+ */
+
+export const DEFAULTS = {
+
+  // ── Gradient background ────────────────────────────────────
+  /** Base colour the gradient is derived from */
+  GRADIENT_COLOR:     '#4d22b3',
+  /** Flow animation speed, slider 1–10 */
+  GRADIENT_SPEED:      2,
+  /** Clockwise rotation off by default */
+  GRADIENT_ROTATION:   true,
+
+  // ── Icon evolution ─────────────────────────────────────────
+  /** Min ms before the first / next icon spawns */
+  SPAWN_DELAY_MIN:     3_000,
+  /** Max ms before the first / next icon spawns */
+  SPAWN_DELAY_MAX:    20_000,
+
+  /** Final rendered icon size in px */
+  ICON_SIZE:              24,
+  /** Half of ICON_SIZE — used for edge/collision math */
+  ICON_HALF:              12,
+
+  /** Movement speed slider default (1–10) */
+  MOVE_SPEED:              5,
+  /** Base px-per-frame at MOVE_SPEED */
+  BASE_SPEED:             0.8,
+  /** Hard cap: max speed = BASE_SPEED × this factor */
+  MAX_SPEED_FACTOR:        2.5,
+
+  /** ms for dot → full-icon grow animation */
+  APPEAR_DURATION:       600,
+
+  /** Probability per frame of a random velocity "kick" (chaotic drift) */
+  DRIFT_CHANCE:           0.02,
+  /** Max |Δv| applied on each drift kick */
+  DRIFT_MAGNITUDE:        0.25,
+
+};