feat: add screen-space weather particle overlay (rain/snow/storm)

Weather type and intensity are already tracked from SMSG_WEATHER, but
only an icon was shown next to the zone name.  This adds a fullscreen
ImDrawList overlay that renders:
- Rain (type 1): diagonal rain streaks proportional to intensity
- Snow (type 2): gently swaying snowflakes with two-tone highlight
- Storm (type 3): heavy rain + dark fog-vignette on screen edges

Particles wrap at screen boundaries and are re-seeded on type or
resolution change.  Delta time is capped at 50 ms to prevent teleporting
after focus loss.  No heap allocations at runtime (static local arrays).

include/ui/game_screen.hpp

@@ -635,6 +635,7 @@ private:
     std::string zoneTextName_;
     std::string lastKnownZoneName_;
     void renderZoneText();
+    void renderWeatherOverlay(game::GameHandler& gameHandler);
 
     // Cooldown tracker
     bool showCooldownTracker_ = false;