feat: add conversation image previews and sharing

docs/frontend.md

@@ -57,10 +57,12 @@ Rules:
 
 The login form exchanges the administrator token for an HttpOnly, SameSite=Strict cookie. API clients may use Bearer tokens. Express applies auth before static dashboard assets, APIs, SSE, and `/media`; `/healthz` is the intentional public exception.
 
-Media may be private conversation content. Preserve authenticated URLs, blur/privacy defaults, explicit reveal behavior, safe new-tab links, and no token leakage into query strings or local storage.
+Media may be private conversation content. Preserve authenticated URLs, blur/privacy defaults, explicit reveal behavior, safe new-tab links, and no token leakage into query strings or local storage. A conversation's source-post thumbnail becomes a keyboard-accessible full-screen preview only after the operator reveals it; the preview closes by button, backdrop, or Escape and restores focus to its trigger.
 
 Exchange-library uploads require an explicit consent checkbox before the raw image request is enabled. Previews remain blurred by default. The management card exposes AI tags, safety level, confidence/review state, enablement, and a two-step delete action. Its progression rail mirrors the actual nonspecific-request selection tiers rather than implying that explicit anatomy requests follow the same sequence.
 
+Conversation reply composers include a collapsed saved-image picker. It lists only server-approved assets for the latest inbound request, keeps thumbnails blurred until revealed, and requires selection plus a dedicated confirmation dialog before sending. The server remains authoritative and may reject a choice if the conversation changes before the rate-limited send executes.
+
 ## Feature structure
 
 Each route is a slice under `features/<name>/` with page/route components, local styles, and feature-specific helpers. Cross-feature visual/domain components live under `components/`; application lifecycle belongs under `app/`; transport belongs under `api/`.