feat(chat): share a chat by link — anonymous read-only /shared/<token> page, share/unshare

frontend/index.html

@@ -134,6 +134,29 @@
         <span class="save-chat-label">Save</span>
       </button>
 
+      <!-- Phase 51 (owner-locked 2026-08-29, `TODO.md` L6): "Share"
+           turns the current conversation into a PUBLIC read-only link —
+           /shared/<token> (a 128-bit uuid4 on the saved_chats row,
+           migration 0009; the anonymous page is task 03). The
+           save-then-share contract: an UNSAVED (unlinked) conversation
+           is saved AND shared in ONE action — app.js POSTs /api/chats
+           with { messages, share: true } (the server sets the token in
+           the same commit) and links the conversation to the created
+           row; a saved (linked) one just POSTs /api/chats/<id>/share
+           (idempotent — the existing token comes back unchanged). On
+           success the ABSOLUTE link is copied to the clipboard; a
+           non-secure (http) homelab origin that rejects the clipboard
+           gets the inline link-field fallback instead (owner-locked —
+           app.js renders .share-link-fallback near the status line).
+           Admin-only — ships HIDDEN exactly like Save (absent-not-
+           hidden, phase 16); app.js reveals it at boot (the same
+           admin-reveal block) and binds the click to shareCurrentChat.
+           Unsharing lives on the History page's Share column (task 04). -->
+      <button type="button" class="share-chat-btn" id="share-chat-btn" aria-label="Share chat" hidden>
+        <svg aria-hidden="true" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.7" stroke-linecap="round" stroke-linejoin="round"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"/><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"/></svg>
+        <span class="share-chat-label">Share</span>
+      </button>
+
       <!-- Phase 49 (2026-08-29, TODO.md L4): the meta row under a brain
            bubble can carry JS-injected actions (app.js) — Tune (admin
            only, phase 15) and Retry (every visitor; the LAST brain