feat: implement textured line clear effects and refine UI alignment

- **Visual Effects**: Upgraded line clear particles to use the game's block texture instead of simple circles, matching the reference web game's aesthetic.
- **Particle Physics**: Tuned particle velocity, gravity, and fade rates for a more dynamic explosion effect.
- **Rendering Integration**: Updated [main.cpp](cci:7://file:///d:/Sites/Work/tetris/src/main.cpp:0:0-0:0) and `GameRenderer` to pass the block texture to the effect system and correctly trigger animations upon line completion.
- **Menu UI**: Fixed [MenuState](cci:1://file:///d:/Sites/Work/tetris/src/states/MenuState.cpp:19:0-19:55) layout calculations to use fixed logical dimensions (1200x1000), ensuring consistent centering and alignment of the logo, buttons, and settings icon across different window sizes.
- **Code Cleanup**: Refactored `PlayingState` to delegate effect triggering to the rendering layer where correct screen coordinates are available.

src/gameplay/core/Game.h

@@ -0,0 +1,116 @@
+
+// Game.h - Core Tetris game logic (board, piece mechanics, scoring events only)
+#pragma once
+#include <array>
+#include <vector>
+#include <random>
+#include <cstdint>
+#include <functional>
+#include <memory>
+#include "../../core/GravityManager.h"
+
+enum PieceType { I, O, T, S, Z, J, L, PIECE_COUNT };
+using Shape = std::array<uint16_t, 4>; // four rotation bitmasks
+
+class Game {
+public:
+    static constexpr int COLS = 10;
+    static constexpr int ROWS = 20;
+    static constexpr int TILE = 28; // logical cell size in pixels (render layer decides use)
+
+    struct Piece { PieceType type{PIECE_COUNT}; int rot{0}; int x{3}; int y{-2}; };
+
+    explicit Game(int startLevel = 0) { reset(startLevel); }
+    void reset(int startLevel = 0);
+
+    // Simulation -----------------------------------------------------------
+    void tickGravity(double frameMs);          // advance gravity accumulator & drop
+    void softDropBoost(double frameMs);        // accelerate fall while held
+    void hardDrop();                           // instant drop & lock
+    void setSoftDropping(bool on) { softDropping = on; } // mark if player holds Down
+    void move(int dx);                         // horizontal move
+    void rotate(int dir);                      // +1 cw, -1 ccw (simple wall-kick)
+    void holdCurrent();                        // swap with hold (once per spawn)
+
+    // Accessors -----------------------------------------------------------
+    const std::array<int, COLS*ROWS>& boardRef() const { return board; }
+    const Piece& current() const { return cur; }
+    const Piece& next() const { return nextPiece; }
+    const Piece& held() const { return hold; }
+    bool canHoldPiece() const { return canHold; }
+    bool isGameOver() const { return gameOver; }
+    bool isPaused() const { return paused; }
+    void setPaused(bool p) { paused = p; }
+    int score() const { return _score; }
+    int lines() const { return _lines; }
+    int level() const { return _level; }
+    int startLevelBase() const { return startLevel; }
+    double elapsed() const { return _elapsedSec; }
+    void addElapsed(double frameMs) { if (!paused) _elapsedSec += frameMs/1000.0; }
+    
+    // Block statistics
+    const std::array<int, PIECE_COUNT>& getBlockCounts() const { return blockCounts; }
+    
+    // Line clearing effects support
+    bool hasCompletedLines() const { return !completedLines.empty(); }
+    const std::vector<int>& getCompletedLines() const { return completedLines; }
+    void clearCompletedLines(); // Actually remove the lines from the board
+    
+    // Sound effect callbacks
+    using SoundCallback = std::function<void(int)>; // Callback for line clear sounds (number of lines)
+    using LevelUpCallback = std::function<void(int)>; // Callback for level up sounds
+    void setSoundCallback(SoundCallback callback) { soundCallback = callback; }
+    void setLevelUpCallback(LevelUpCallback callback) { levelUpCallback = callback; }
+
+    // Shape helper --------------------------------------------------------
+    static bool cellFilled(const Piece& p, int cx, int cy);
+
+    // Gravity tuning accessors (public API for HUD/tuning)
+    void setGravityGlobalMultiplier(double m) { gravityGlobalMultiplier = m; }
+    double getGravityGlobalMultiplier() const;
+    double getGravityMs() const;
+    void setLevelGravityMultiplier(int level, double m);
+
+private:
+    std::array<int, COLS*ROWS> board{}; // 0 empty else color index
+    Piece cur{}, hold{}, nextPiece{};   // current, held & next piece
+    bool canHold{true};
+    bool paused{false};
+    std::vector<PieceType> bag;         // 7-bag randomizer
+    std::mt19937 rng{ std::random_device{}() };
+    std::array<int, PIECE_COUNT> blockCounts{}; // Count of each piece type used
+
+    int _score{0};
+    int _lines{0};
+    int _level{1};
+    double gravityMs{800.0};
+    double fallAcc{0.0};
+    double _elapsedSec{0.0};
+    bool gameOver{false};
+    int startLevel{0};
+    bool softDropping{false}; // true while player holds Down key
+    
+    // Line clearing support
+    std::vector<int> completedLines; // Rows that are complete and ready for effects
+    
+    // Sound effect callbacks
+    SoundCallback soundCallback;
+    LevelUpCallback levelUpCallback;
+    // Gravity tuning -----------------------------------------------------
+    // Global multiplier applied to all level timings (use to slow/speed whole-game gravity)
+    double gravityGlobalMultiplier{1.0};
+    // Gravity manager encapsulates frames table, multipliers and conversions
+    GravityManager gravityMgr;
+    // Backwards-compatible accessors (delegate to gravityMgr)
+    double computeGravityMsForLevel(int level) const;
+
+    // Internal helpers ----------------------------------------------------
+    void refillBag();
+    void spawn();
+    bool collides(const Piece& p) const;
+    void lockPiece();
+    int checkLines(); // Find completed lines and store them
+    void actualClearLines(); // Actually remove lines from board
+    bool tryMoveDown(); // one-row fall; returns true if moved
+    // Gravity tuning helpers (public API declared above)
+};