Add chat settings modal & reply previews

Introduce a full ChatSettingsModal component for managing group chats (rename/icon upload, add/remove members, toggle moderators, leave/delete) and wire it into ChatRoomPage with a settings button. Add reply preview support end-to-end: new ReplyToSerializer and members_detail in backend serializers, updated frontend Message model (reply_to now contains preview object), and UI changes to Message to render reply excerpts. Improve socket handling to attach reply previews when available. Tweak backend Dockerfile to optionally install Windows/corporate CA bundle only if present and move pip install after copying source. Add Czech translations and small tooling/.claude config enhancements.

backend/Dockerfile

@@ -2,10 +2,6 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-# Trust Windows/corporate root CAs before any network operations
-COPY certs/windows-ca-bundle.crt /usr/local/share/ca-certificates/windows-ca-bundle.crt
-RUN update-ca-certificates
-
 # Install system dependencies including Node.js for yt-dlp JavaScript runtime
 RUN apt-get update && apt-get install -y --no-install-recommends \
     weasyprint \
@@ -18,13 +14,18 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libmagic1 \
     && curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \
-    && update-ca-certificates \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
-
 COPY . .
 
+# Trust Windows/corporate root CAs if present (optional, no-op when certs/ is absent)
+RUN test -f certs/windows-ca-bundle.crt \
+    && install -m 644 certs/windows-ca-bundle.crt /usr/local/share/ca-certificates/windows-ca-bundle.crt \
+    && update-ca-certificates \
+    || true
+
+RUN pip install --no-cache-dir -r requirements.txt
+
 EXPOSE 8000