fix: differentiate Conex (linkedEntryIds) vs Inchide (threadParentId) semantics

- Conex button now adds to linkedEntryIds (for clarificari/solicitari)
  instead of setting threadParentId
- Inchide button sets threadParentId (direct reply) + auto-closes parent
- Fix Sterge button persistence bug: threadParentId now saves as empty
  string instead of undefined (which was stripped in JSON serialization)
- Card headers: "Conex cu X" (amber) vs "Raspuns la X" (blue + green)
- Add conexTo prop to RegistryEntryForm for linked entry pre-fill

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/modules/registratura/components/registry-table.tsx

@@ -483,7 +483,7 @@ export function RegistryTable({
                             e.stopPropagation();
                             onReply(entry);
                           }}
-                          title="Conex — creeaza raspuns"
+                          title="Conex — clarificare / solicitare"
                         >
                           <Reply className="h-3.5 w-3.5" />
                         </Button>