Add mouse support to the TUI (tmux-style tile resize)

Enable mouse in the Bubble Tea program. In the layout grid editor:
click a cell to assign a camera, or press on a tile and drag to
resize its span (grid-snapped). Click-to-select on the menu/list
screens and the camera picker. Works over SSH; edits the config,
so it never fights the daemon geometry.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tui/mouse_test.go

@@ -0,0 +1,33 @@
+package tui
+
+import (
+	"testing"
+
+	"github.com/lwoodard/rtsp-streamer/internal/config"
+)
+
+func TestCellAtMouse(t *testing.T) {
+	m := &model{
+		cfg:        &config.Config{Layouts: []config.Layout{{Name: "g", Grid: "3x3"}}},
+		editLayout: 0,
+	}
+	cases := []struct {
+		x, y          int
+		wantCol, wRow int
+		ok            bool
+	}{
+		{1, 3, 0, 0, true},   // first content row, first cell
+		{12, 3, 1, 0, true},  // second column (x past the divider at 11)
+		{1, 6, 0, 1, true},   // second grid row
+		{23, 9, 2, 2, true},  // third column, third row
+		{1, 1, 0, 0, false},  // in the title/blank header area
+		{1, 12, 0, 0, false}, // below the 3x3 grid
+	}
+	for _, tc := range cases {
+		col, row, ok := m.cellAtMouse(tc.x, tc.y)
+		if ok != tc.ok || (ok && (col != tc.wantCol || row != tc.wRow)) {
+			t.Errorf("cellAtMouse(%d,%d) = (%d,%d,%v), want (%d,%d,%v)",
+				tc.x, tc.y, col, row, ok, tc.wantCol, tc.wRow, tc.ok)
+		}
+	}
+}