feat: add user display profiles and fix web context menu UX

backend:

- add required user name and optional bio fields

- extend auth/register and user schemas/services with name/bio

- add alembic migration 0006 with safe backfill name=username

- compute per-user chat display_title for private chats

- keep Saved Messages delete-for-all protections

web:

- registration now includes name

- add profile edit modal (name/username/bio/avatar url)

- show private chat names via display_title

- fix context menus to open near cursor with viewport clamping

- stabilize +/close floating button to remove visual jump

web/src/components/MessageList.tsx

@@ -70,7 +70,8 @@ export function MessageList() {
                 }`}
                 onContextMenu={(e) => {
                   e.preventDefault();
-                  setCtx({ x: e.clientX, y: e.clientY, messageId: message.id });
+                  const pos = getSafeContextPosition(e.clientX, e.clientY, 160, 108);
+                  setCtx({ x: pos.x, y: pos.y, messageId: message.id });
                 }}
               >
                 {message.forwarded_from_message_id ? (
@@ -147,3 +148,13 @@ function renderStatus(status: string | undefined): string {
   if (status === "read") return "✓✓";
   return "✓";
 }
+
+function getSafeContextPosition(x: number, y: number, menuWidth: number, menuHeight: number): { x: number; y: number } {
+  const pad = 8;
+  const cursorOffset = 4;
+  const wantedX = x + cursorOffset;
+  const wantedY = y + cursorOffset;
+  const safeX = Math.min(Math.max(pad, wantedX), window.innerWidth - menuWidth - pad);
+  const safeY = Math.min(Math.max(pad, wantedY), window.innerHeight - menuHeight - pad);
+  return { x: safeX, y: safeY };
+}