render message reply threads in dashboard and per-agent inbox

- MessageEvent and DashboardEvent Sent/Delivered now carry id and in_reply_to
- broker.send() includes last_insert_rowid in the emitted event
- recent_all() and recv_batch() include id and in_reply_to from the DB
- deliver_reminders_batch() tracks per-row rowids within the transaction
- dashboard message flow: reply rows are indented with a border-left and a
  clickable '↳ reply' tag that scroll-jumps + briefly highlights the parent
- per-agent inbox: reply messages get a '↳ reply ·' prefix and indent

Closes #26

hive-c0re/src/dashboard_events.rs

@@ -40,10 +40,14 @@ pub enum DashboardEvent {
     /// appear in this list, everything else stays plain text.
     Sent {
         seq: u64,
+        /// Broker row id. Allows the dashboard to track reply threads.
+        id: i64,
         from: String,
         to: String,
         body: String,
         at: i64,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        in_reply_to: Option<i64>,
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         file_refs: Vec<String>,
     },
@@ -51,10 +55,14 @@ pub enum DashboardEvent {
     /// `file_refs` is the same shape as `Sent`.
     Delivered {
         seq: u64,
+        /// Broker row id. Allows the dashboard to track reply threads.
+        id: i64,
         from: String,
         to: String,
         body: String,
         at: i64,
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        in_reply_to: Option<i64>,
         #[serde(default, skip_serializing_if = "Vec::is_empty")]
         file_refs: Vec<String>,
     },