fix(odin): clear composer on send + readable selection in user bubble

- the sent message now leaves the input the moment the optimistic bubble
  appears (restored along with attachments if the request fails)
- text selected in the primary-filled user bubble inverts to
  white-on-primary instead of vanishing into the global ::selection style

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/admin/components/odin/OdinThread.tsx

@@ -196,6 +196,15 @@ export default function OdinThread({
                 borderRadius: 2,
                 boxShadow: 1,
                 bgcolor: isUser ? "primary.main" : "background.paper",
+                // The global ::selection is primary-on-white — invisible on
+                // this primary-filled bubble; invert it so selected/copied
+                // text stays readable.
+                ...(isUser && {
+                  "& ::selection": {
+                    backgroundColor: "common.white",
+                    color: "primary.main",
+                  },
+                }),
               }}
             >
               {/* Tools the assistant consulted for this turn (Phase 2a). */}