fix(render): code quality cleanup

Magic number elimination:
- Create protocol_constants.hpp, warden_constants.hpp,
  render_constants.hpp, ui_constants.hpp
- Replace ~55 magic numbers across game_handler, warden_handler,
  m2_renderer_render

Reduce nesting depth:
- Extract 5 parseEffect* methods from handleSpellLogExecute
  (max indent 52 → 16 cols)
- Extract resolveSpellSchool/playSpellCastSound/playSpellImpactSound
  from 3× duplicate audio blocks in handleSpellGo
- Flatten SMSG_INVENTORY_CHANGE_FAILURE with early-return guards
- Extract drawScreenEdgeVignette() for 3 duplicate vignette blocks

DRY extract patterns:
- Replace 12 compound expansion checks with isPreWotlk() across
  movement_handler (9), chat_handler (1), social_handler (1)

const to constexpr:
- Promote 23+ static const arrays/scalars to static constexpr across
  12 source files

Error handling:
- Convert PIN auth from exceptions to std::optional<PinProof>
- Add [[nodiscard]] to 15+ initialize/parse methods
- Wrap ~20 unchecked initialize() calls with LOG_WARNING/LOG_ERROR

Signed-off-by: Pavel Okhlopkov <pavel.okhlopkov@flant.com>

include/ui/ui_constants.hpp

@@ -0,0 +1,68 @@
+#pragma once
+
+#include <cstdint>
+
+// UI-layer constants: colors, layout sizes, effect timing.
+
+namespace wowee {
+namespace ui {
+
+// ---------------------------------------------------------------------------
+// Target selection-circle difficulty colours (WoW-canonical)
+// ---------------------------------------------------------------------------
+// Stored as {R, G, B} in [0,1].  Alpha is set by the caller.
+struct Colour3f { float r, g, b; };
+
+constexpr Colour3f SEL_COLOR_DEFAULT_YELLOW = {1.0f, 1.0f, 0.3f};
+constexpr Colour3f SEL_COLOR_RED            = {1.0f, 0.1f, 0.1f};
+constexpr Colour3f SEL_COLOR_ORANGE         = {1.0f, 0.5f, 0.1f};
+constexpr Colour3f SEL_COLOR_YELLOW         = {1.0f, 1.0f, 0.1f};
+constexpr Colour3f SEL_COLOR_GREEN          = {0.3f, 1.0f, 0.3f};
+constexpr Colour3f SEL_COLOR_GREY           = {0.6f, 0.6f, 0.6f};
+constexpr Colour3f SEL_COLOR_DEAD           = {0.5f, 0.5f, 0.5f};
+
+// Level-diff thresholds that select colours above.
+constexpr int MOB_LEVEL_DIFF_RED    = 10;   // ≥ 10 levels above player → red
+constexpr int MOB_LEVEL_DIFF_ORANGE = 5;    // ≥ 5      → orange
+constexpr int MOB_LEVEL_DIFF_YELLOW_FLOOR = -2; // ≥ -2 → yellow, else green
+
+// Selection circle world-unit bounds
+constexpr float SEL_CIRCLE_MIN_RADIUS = 0.8f;
+constexpr float SEL_CIRCLE_MAX_RADIUS = 8.0f;
+
+// ---------------------------------------------------------------------------
+// Damage flash / vignette effect
+// ---------------------------------------------------------------------------
+constexpr float DAMAGE_FLASH_FADE_SPEED      = 2.0f;   // alpha units/sec
+constexpr float DAMAGE_FLASH_ALPHA_SCALE     = 100.0f;  // multiplier
+constexpr uint8_t DAMAGE_FLASH_RED_CHANNEL   = 200;     // IM_COL32 R channel
+constexpr float DAMAGE_VIGNETTE_THICKNESS    = 0.12f;   // fraction of screen
+
+// ---------------------------------------------------------------------------
+// Low-health pulsing vignette
+// ---------------------------------------------------------------------------
+constexpr float LOW_HEALTH_THRESHOLD_PCT      = 0.20f;  // start at 20% HP
+constexpr float LOW_HEALTH_PULSE_FREQUENCY    = 9.4f;   // angular speed (~1.5 Hz)
+constexpr float LOW_HEALTH_MAX_ALPHA          = 90.0f;
+constexpr float LOW_HEALTH_VIGNETTE_THICKNESS = 0.15f;
+
+// ---------------------------------------------------------------------------
+// Level-up flash overlay
+// ---------------------------------------------------------------------------
+constexpr float LEVELUP_FLASH_FADE_SPEED      = 1.0f;   // alpha units/sec
+constexpr float LEVELUP_FLASH_ALPHA_SCALE     = 160.0f;
+constexpr float LEVELUP_VIGNETTE_THICKNESS    = 0.18f;
+constexpr float LEVELUP_TEXT_FONT_SIZE        = 28.0f;
+
+// ---------------------------------------------------------------------------
+// Ghost / death state
+// ---------------------------------------------------------------------------
+constexpr float GHOST_OPACITY = 0.5f;
+
+// ---------------------------------------------------------------------------
+// Click / interaction thresholds
+// ---------------------------------------------------------------------------
+constexpr float CLICK_THRESHOLD_PX = 5.0f;
+
+} // namespace ui
+} // namespace wowee