fix(chat): stop placeholder ghosting in chat composer (#65)

`TextEditor`'s NSTextView surfaces a typed glyph one frame before
the SwiftUI binding propagates, so the bare `if text.isEmpty`
overlay rendered the translucent placeholder text directly on top
of the just-typed character — the "behind or around" ghost the
reporter described.

Two mitigations:

- Pin an opaque `ScarfColor.backgroundSecondary` rect behind the
  placeholder Text. During any single-frame binding lag the user
  now sees a clean placeholder rather than layered glyphs.
- Switch the conditional to `.opacity(text.isEmpty ? 1 : 0)` so the
  view tree stays stable per keystroke. Pairs with the composer
  perf fix from #67.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scarf/scarf/Features/Chat/Views/RichChatInputBar.swift

@@ -108,7 +108,23 @@ struct RichChatInputBar: View {
                             )
                     )
                     .overlay(alignment: .topLeading) {
-                        if text.isEmpty {
+                        // Placeholder ghosting (#65): TextEditor's
+                        // NSTextView updates the visible glyphs a frame
+                        // before the SwiftUI binding propagates, so a
+                        // bare `if text.isEmpty` overlay renders the
+                        // translucent placeholder text on top of the
+                        // just-typed character — visible as a "behind
+                        // or around" ghost. Two mitigations:
+                        //
+                        //   1. Pin an opaque rectangle behind the
+                        //      placeholder text. During any single-
+                        //      frame lag the user sees a clean
+                        //      placeholder, never layered glyphs.
+                        //   2. Use `.opacity(...)` instead of an `if`.
+                        //      Keeps the view tree stable per
+                        //      keystroke (removes the per-keystroke
+                        //      view-mutation churn the composer was
+                        //      already paying for).
                         Text(supportsImagePrompts
                              ? "Message Hermes…  /  for commands · drag images to attach"
                              : "Message Hermes…  /  for commands")
@@ -116,9 +132,10 @@ struct RichChatInputBar: View {
                             .foregroundStyle(ScarfColor.foregroundFaint)
                             .padding(.horizontal, 14)
                             .padding(.vertical, 10)
+                            .background(ScarfColor.backgroundSecondary)
+                            .opacity(text.isEmpty ? 1 : 0)
                             .allowsHitTesting(false)
                     }
-                    }
                     // Drag-drop image attachments. Receives both file URLs
                     // (from Finder) and raw image bitmap data (from
                     // screenshot tools that drop tiff/png directly).