overlay: preserve explicit newlines in bubbles (raw \n to <br>)

Qt rich text collapses raw newlines to whitespace; convert before render.

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Co-Authored-By: Letta Code <noreply@letta.com>

src/overlay.zig

@@ -303,6 +303,10 @@ fn toHtml(allocator: std.mem.Allocator, text: []const u8) ![]u8 {
                 try out.append(allocator, c);
                 i += 1;
             }
+        } else if (c == '\n') {
+            // Rich text collapses raw newlines — explicit <br> survives.
+            try out.appendSlice(allocator, "<br>");
+            i += 1;
         } else {
             try out.append(allocator, c);
             i += 1;