feat: paint theme background across the editor canvas

@@ -267,22 +267,6 @@ }
 
 func (a *App) topPad() int { return canvasTopPad }
 
-// indentLines prefixes each non-empty line with n spaces (blank rows stay blank,
-// so the trailing newline of a sub-view does not become a spaces-only row).
-func indentLines(s string, n int) string {
-	if n <= 0 {
-		return s
-	}
-	pad := strings.Repeat(" ", n)
-	lines := strings.Split(s, "\n")
-	for i, ln := range lines {
-		if ln != "" {
-			lines[i] = pad + ln
-		}
-	}
-	return strings.Join(lines, "\n")
-}
-
 func (a *App) setSize(w, h int) {
 	a.width = w
 	a.height = h
@@ -370,14 +354,29 @@ 		body = a.preview.View()
 	} else {
 		body = a.editor.View()
 	}
-	body = indentLines(body, a.leftMargin())
-	if a.topPad() > 0 {
-		body = strings.Repeat("\n", a.topPad()) + body
+	return a.paintCanvas(body) + a.statusBar()
+}
+
+// paintCanvas centers body in the theme's paper: a top pad, then each body line
+// indented by the left margin, every line filled to the full terminal width with
+// the theme background so margins and empty space carry the theme color rather
+// than the terminal default. The result is exactly height-1 rows (the status bar
+// is the final row) and always ends with a trailing newline before it.
+func (a *App) paintCanvas(body string) string {
+	bg := lipgloss.NewStyle().Background(a.theme.Background).Width(maxInt(a.width, 1))
+	lm := strings.Repeat(" ", a.leftMargin())
+	rows := make([]string, 0, a.height)
+	for p := 0; p < a.topPad(); p++ {
+		rows = append(rows, bg.Render(""))
 	}
-	if !strings.HasSuffix(body, "\n") {
-		body += "\n"
+	for _, ln := range strings.Split(body, "\n") {
+		if ln == "" {
+			rows = append(rows, bg.Render(""))
+		} else {
+			rows = append(rows, bg.Render(lm+ln))
+		}
 	}
-	return body + a.statusBar()
+	return strings.Join(rows, "\n") + "\n"
 }
 
 func (a *App) statusBar() string {

@@ -7,9 +7,25 @@ 	"strings"
 	"testing"
 
 	"glint/internal/config"
+	"glint/internal/theme"
 
 	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
 )
+
+func TestCanvasPaintsFullWidthBackground(t *testing.T) {
+	a := newApp()
+	a.theme = theme.FlexokiDark()
+	a.editor.SetTheme(theme.FlexokiDark())
+	a.Update(tea.WindowSizeMsg{Width: 100, Height: 12})
+	a.editor.SetContent([]byte("hi"))
+	out := a.View()
+	for i, ln := range strings.Split(out, "\n") {
+		if w := lipgloss.Width(ln); w != 100 {
+			t.Errorf("line %d visible width = %d, want 100 (full-width background fill)", i, w)
+		}
+	}
+}
 
 func newApp() *App {
 	return New(config.Default())

@@ -3,6 +3,8 @@
 import (
 	"testing"
 
+	"glint/internal/theme"
+
 	tea "github.com/charmbracelet/bubbletea"
 )
 
@@ -271,3 +273,18 @@ 	if ci < e.Scroll || ci >= e.Scroll+e.Height {
 		t.Errorf("cursor visual row %d not in viewport [%d,%d)", ci, e.Scroll, e.Scroll+e.Height)
 	}
 }
+
+func TestBuildVisualSpansCarryThemeBackground(t *testing.T) {
+	e := New()
+	th := theme.FlexokiDark()
+	e.SetTheme(th)
+	e.Lines = []string{"# Heading **bold**", "plain text", ""}
+	for _, vr := range e.buildVisual() {
+		for _, sp := range vr.spans {
+			if sp.Style.GetBackground() != th.Background {
+				t.Errorf("span %q background = %v, want theme background %v",
+					sp.Text, sp.Style.GetBackground(), th.Background)
+			}
+		}
+	}
+}

@@ -1,5 +1,7 @@
 package editor
 
+import "github.com/charmbracelet/lipgloss"
+
 // segment is one visual row of a logical line: a contiguous rune slice plus the
 // rune offset (start column) within the logical line where it begins.
 type segment struct {
@@ -74,7 +76,8 @@ 	spans  []Span
 }
 
 // buildVisual wraps every logical line into visual rows, slicing each line's
-// styled spans to the segment ranges.
+// styled spans to the segment ranges. Every span is given the theme background
+// so glyphs sit on the theme's paper rather than the terminal default.
 func (e *Editor) buildVisual() []vrow {
 	all := ScanLines(e.Lines, e.theme)
 	var rows []vrow
@@ -85,11 +88,20 @@ 			rows = append(rows, vrow{
 				logRow: li,
 				start:  s.start,
 				runes:  n,
-				spans:  sliceSpans(all[li], s.start, s.start+n),
+				spans:  withBackground(sliceSpans(all[li], s.start, s.start+n), e.theme.Background),
 			})
 		}
 	}
 	return rows
+}
+
+// withBackground stamps the theme background onto every span's style so the
+// rendered text has no terminal-default gaps between or under glyphs.
+func withBackground(spans []Span, bg lipgloss.Color) []Span {
+	for i := range spans {
+		spans[i].Style = spans[i].Style.Background(bg)
+	}
+	return spans
 }
 
 // cursorVIndex returns the index of the visual row holding the cursor. A cursor