chat: enforce shared text policy at UI, sign point, and gossip ingress

Chat-hardening plan Phase 1. The chat body policy (2,000-char + 8 KiB
ceilings, single-pass control/whitespace normalization) moves from the UI
layer into src/sanitize.rs and is now enforced at every trust boundary:
cap_chat_input bounds the live input (oversized paste), the gossip sign
point re-sanitizes so non-UI callers can't bypass policy, and gossip
ingress rejects oversized raw text before sanitizing (admit_chat_text)
and drops messages with neither visible text nor an attachment. The
incoming chat author label now uses the strict name sanitizer until
Phase 2 roster-binds it. +8 tests (532 lib green).

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

src/network/gossip.rs

@@ -819,6 +819,21 @@ impl RoomState for IrohGossipState {
                                             a.name = crate::files::sanitize_filename(&a.name);
                                             Some(a)
                                         });
+                                        // Chat text policy at INGRESS: reject raw text
+                                        // over the byte ceiling before spending any
+                                        // sanitize work on it (a compliant sender
+                                        // sanitizes before signing), and drop a message
+                                        // with neither visible text nor an attachment.
+                                        let Some(text) = crate::sanitize::admit_chat_text(
+                                            &text,
+                                            attachment.is_some(),
+                                        ) else {
+                                            crate::log_msg(&format!(
+                                                "Dropped out-of-policy chat from author={:?} (oversized or empty)",
+                                                payload.author
+                                            ));
+                                            continue;
+                                        };
                                         let _ = event_tx
                                             .send(RoomEvent::ChatMessage {
                                                 from: payload.author,
@@ -966,6 +981,14 @@ impl RoomState for IrohGossipState {
         text: String,
         attachment: Option<crate::files::ChatAttachment>,
     ) -> Result<(), NetError> {
+        // Enforce the chat text policy at the SIGN point, not only in the UI, so
+        // a future non-UI caller can't sign an out-of-policy body (chat-hardening
+        // plan Phase 1). Idempotent over the UI's own sanitize pass.
+        let text = crate::sanitize::sanitize_chat(&text);
+        if text.is_empty() && attachment.is_none() {
+            // Nothing visible to send — not an error, just nothing to do.
+            return Ok(());
+        }
         let name = {
             let guard = self.self_state.lock().unwrap();
             match guard.as_ref() {