Add Connect 4 multiplayer game server

Real-time two-player Connect 4 using Via framework with:
- Game creation and invite links
- SSE-based live updates for both players
- Win detection with animated highlighting
- Session-based nickname persistence

ui/board.go

@@ -0,0 +1,60 @@
+package ui
+
+import (
+	"github.com/ryanhamamura/c4/game"
+	"github.com/ryanhamamura/via/h"
+)
+
+// ColumnClickFn returns an h.H onClick attribute for a given column index
+type ColumnClickFn func(col int) h.H
+
+func BoardComponent(g *game.Game, columnClick ColumnClickFn, myColor int) h.H {
+	var cols []h.H
+
+	for col := 0; col < 7; col++ {
+		var cells []h.H
+		for row := 0; row < 6; row++ {
+			cellColor := g.Board[row][col]
+			isWinning := g.IsWinningCell(row, col)
+			cells = append(cells, Cell(cellColor, isWinning))
+		}
+
+		// Column is clickable only if it's player's turn and game is in progress
+		canClick := g.Status == game.StatusInProgress && g.CurrentTurn == myColor
+		cols = append(cols, Column(col, cells, columnClick, canClick))
+	}
+
+	boardAttrs := []h.H{h.Class("board")}
+	boardAttrs = append(boardAttrs, cols...)
+	return h.Div(boardAttrs...)
+}
+
+func Column(colIdx int, cells []h.H, columnClick ColumnClickFn, canClick bool) h.H {
+	class := "column"
+	if canClick {
+		class += " clickable"
+	}
+
+	attrs := []h.H{h.Class(class)}
+
+	if canClick && columnClick != nil {
+		attrs = append(attrs, columnClick(colIdx))
+	}
+
+	attrs = append(attrs, cells...)
+	return h.Div(attrs...)
+}
+
+func Cell(color int, isWinning bool) h.H {
+	class := "cell"
+	switch color {
+	case 1:
+		class += " red"
+	case 2:
+		class += " yellow"
+	}
+	if isWinning {
+		class += " winning"
+	}
+	return h.Div(h.Class(class))
+}