fix(chat): clip placeholder to TextEditor bounds and clear it on focus

Two related bugs in the Mac chat composer's placeholder overlay:

* The "Message Hermes… / for commands · drag images to attach" hint had
  no width constraint, so on narrower window geometries it visibly
  overflowed past the rounded TextEditor boundary. Add `lineLimit(1)`,
  `truncationMode(.tail)`, and `frame(maxWidth: .infinity, alignment:
  .leading)` so it ellipsizes inside the field instead.
* The opacity formula `text.isEmpty ? 1 : 0` only hid the placeholder
  once content was typed, not when the field gained focus. Standard
  NSTextField / UITextField semantics clear the placeholder on focus.
  Switch to `(text.isEmpty && !isFocused) ? 1 : 0` so the hint
  disappears the moment the user clicks into the field.

The opaque-background ghosting mitigation from #65 is preserved
unchanged.

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

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

@@ -114,7 +114,7 @@ struct RichChatInputBar: View {
                         // 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:
+                        // or around" ghost. Three mitigations:
                         //
                         //   1. Pin an opaque rectangle behind the
                         //      placeholder text. During any single-
@@ -125,15 +125,29 @@ struct RichChatInputBar: View {
                         //      keystroke (removes the per-keystroke
                         //      view-mutation churn the composer was
                         //      already paying for).
+                        //   3. Constrain to a single line with
+                        //      `frame(maxWidth: .infinity)` and
+                        //      `truncationMode(.tail)` so the long-form
+                        //      hint can't escape the rounded
+                        //      TextEditor bounds when the sidebar /
+                        //      detail-pane geometry compresses the
+                        //      composer (was visibly overflowing).
                         Text(supportsImagePrompts
                              ? "Message Hermes…  /  for commands · drag images to attach"
                              : "Message Hermes…  /  for commands")
                             .scarfStyle(.body)
                             .foregroundStyle(ScarfColor.foregroundFaint)
+                            .lineLimit(1)
+                            .truncationMode(.tail)
+                            .frame(maxWidth: .infinity, alignment: .leading)
                             .padding(.horizontal, 14)
                             .padding(.vertical, 10)
                             .background(ScarfColor.backgroundSecondary)
-                            .opacity(text.isEmpty ? 1 : 0)
+                            // Hide once the field has any content OR
+                            // the user is actively focused — matches
+                            // standard NSTextField / UITextField
+                            // placeholder semantics.
+                            .opacity((text.isEmpty && !isFocused) ? 1 : 0)
                             .allowsHitTesting(false)
                     }
                     // Drag-drop image attachments. Receives both file URLs