feat(chat): add Send button to agent composers (mobile fix)

Soft keyboards have no reliable Enter-to-send, so chat was unsendable on
mobile browsers. Add an optional themed Send button wired through
wireAgentChat (Enter-to-send kept for desktop), applied to the Companion
rail, Yerin, and Little Blue composers. Blackflame-styled, flex-row layout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/components/rightrail.js

@@ -22,12 +22,13 @@ export async function renderRightrail(root) {
 
   const log = el('div', { class: 'rail-log' });
   const input = el('textarea', { class: 'rail-input', rows: 1, placeholder: 'Ask the companion…' });
+  const sendBtn = el('button', { class: 'rail-send', type: 'button', title: 'Send', 'aria-label': 'Send' }, '➤');
   const header = el('div', { class: 'rail-hd' },
     el('span', { class: 'who' }, '◆ Companion'),
     el('span', { class: 'chev', onclick: toggle, title: 'Collapse' }, '⟩'));
 
   mount(root, el('div', { class: 'rail-toggle', onclick: toggle, title: 'Companion' }, 'CRADLE'),
-    el('div', { class: 'rail-chat' }, header, log, el('div', { class: 'rail-inputwrap' }, input)));
+    el('div', { class: 'rail-chat' }, header, log, el('div', { class: 'rail-inputwrap' }, input, sendBtn)));
 
   // (Re)wire the chat whenever the active Space changes.
   async function initChat(spaceId) {
@@ -36,7 +37,7 @@ export async function renderRightrail(root) {
       return;
     }
     const chat = wireAgentChat({
-      logEl: log, inputEl: input,
+      logEl: log, inputEl: input, sendBtnEl: sendBtn,
       historyUrl: `/api/spaces/${spaceId}/companion`,
       turnUrl: `/api/spaces/${spaceId}/companion/turn`,
       agentName: 'Companion', showDrafts: true, toolLabels: COMPANION_LABELS,