fix: banking shell screen rendering artifacts and transfer panic

Fix rendering issues where content from previous screens bled through
when switching between views of different heights/widths:

- Pad every line to full terminal width (ANSI-aware) so shorter lines
  overwrite leftover content from previous renders
- Track terminal height via WindowSizeMsg and pad between content and
  footer to fill the screen
- Send tea.ClearScreen on all screen transitions for height changes
- Fix panic in transfer completion when routing number is < 4 chars

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/shell/banking/banking_test.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/lipgloss"
 
 	"git.t-juice.club/torjus/oubliette/internal/shell"
 	"git.t-juice.club/torjus/oubliette/internal/storage"
@@ -520,7 +521,7 @@ func TestConfigString(t *testing.T) {
 }
 
 func TestScreenFrame(t *testing.T) {
-	frame := screenFrame("TESTBANK", "TB-0001", "NORTHEAST", "content here")
+	frame := screenFrame("TESTBANK", "TB-0001", "NORTHEAST", "content here", 0)
 	if !strings.Contains(frame, "TESTBANK FEDERAL RESERVE SYSTEM") {
 		t.Error("frame should contain bank name in header")
 	}
@@ -531,3 +532,29 @@ func TestScreenFrame(t *testing.T) {
 		t.Error("frame should contain the content")
 	}
 }
+
+func TestScreenFramePadsLines(t *testing.T) {
+	frame := screenFrame("TESTBANK", "TB-0001", "NE", "short\n", 0)
+	for i, line := range strings.Split(frame, "\n") {
+		w := lipgloss.Width(line)
+		if w > 0 && w < termWidth {
+			t.Errorf("line %d has visual width %d, want at least %d: %q", i, w, termWidth, line)
+		}
+	}
+}
+
+func TestScreenFramePadsToHeight(t *testing.T) {
+	short := screenFrame("TESTBANK", "TB-0001", "NE", "line1\nline2\n", 30)
+	lines := strings.Count(short, "\n")
+	// Total newlines should be at least height-1 (since the last line has no trailing newline).
+	if lines < 29 {
+		t.Errorf("padded frame has %d newlines, want at least 29 for height=30", lines)
+	}
+
+	// Without height, no padding.
+	noPad := screenFrame("TESTBANK", "TB-0001", "NE", "line1\nline2\n", 0)
+	noPadLines := strings.Count(noPad, "\n")
+	if noPadLines >= 29 {
+		t.Errorf("unpadded frame has %d newlines, should be much less than 29", noPadLines)
+	}
+}