Task #470: Remove popup color ring and square its corners

User asked to drop the colored ring around the floating new-note popup
(added in #468) and replace the rounded corners with square ones for a
cleaner, more formal notification look.

Changes (artifacts/tx-os/src/components/notes/incoming-note-popup.tsx):
- Removed `ring-4 ${ringClass}` from the outer card div so no colored
  outline appears around the popup.
- Replaced `rounded-2xl` with `rounded-none` so the card has square
  90° corners on all sides. `overflow-hidden` is preserved so the
  inner header / body / action bar still clip cleanly to the
  rectangle.
- Dropped the now-unused `colorRingStrong` import and its derived
  `ringClass` local. The shared helper itself stays in
  `notes-api.ts` in case future surfaces want it.
- Kept the neutral `shadow-2xl` so the card still separates from the
  background, and kept the avatar / ping color tinting from #468.

Validation:
- `pnpm --filter @workspace/tx-os exec tsc --noEmit` passes.
- No other functional changes (drag, dismiss, mark read, reply, open,
  checklist toggle, queue counter, RTL all untouched).
- Pre-existing `test` workflow failures (executive-meetings PDF,
  notes-share, groups-crud) are unchanged and out of scope.

artifacts/tx-os/src/components/notes/incoming-note-popup.tsx

@@ -13,7 +13,6 @@ import { Button } from "@/components/ui/button";
 import {
   colorBg,
   colorAccent,
-  colorRingStrong,
   colorAvatarBg,
   colorAvatarRing,
   colorPing,
@@ -351,7 +350,6 @@ export function IncomingNotePopup() {
   // ring + pink ping + pink avatar tint, etc.) instead of the previous
   // fixed amber chrome that clashed with non-amber notes.
   const noteColor = current.color;
-  const ringClass = colorRingStrong(noteColor);
   const avatarBgClass = colorAvatarBg(noteColor);
   const avatarRingClass = colorAvatarRing(noteColor);
   const pingClass = colorPing(noteColor);
@@ -478,7 +476,7 @@ export function IncomingNotePopup() {
       }}
     >
         <div
-          className={`rounded-2xl ring-4 ${ringClass} shadow-2xl overflow-hidden transition-transform duration-200 ${
+          className={`rounded-none shadow-2xl overflow-hidden transition-transform duration-200 ${
             noteColor === "default"
               ? "bg-background"
               : colorBg(noteColor)