feat: Add SSH remote execution for multi-host Claude sessions

- Backend: SSH execution via spawn() with -T flag for JSON streaming
- Backend: PATH setup for non-login shells on remote hosts
- Backend: History loading via SSH (tail -n 2000 for large files)
- Frontend: Host selector UI with colored buttons in Sidebar
- Frontend: Auto-select first project when host changes
- Frontend: Pass host parameter to history and session APIs
- Docker: Install openssh-client, mount SSH keys

Enables running Claude sessions on remote hosts via SSH while
viewing them through the web UI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,13 +1,17 @@
 FROM node:20-slim
 
+# Install SSH client for remote execution
+RUN apt-get update && apt-get install -y openssh-client && rm -rf /var/lib/apt/lists/*
+
 # Create app directory
 WORKDIR /app
 
-# Create node user home structure for claude
+# Create node user home structure for claude and ssh
 RUN mkdir -p /home/node/.local/bin && \
     mkdir -p /home/node/.local/share/claude && \
     mkdir -p /home/node/.claude && \
     mkdir -p /home/node/.config/claude && \
+    mkdir -p /home/node/.ssh && \
     chown -R node:node /home/node
 
 # Create symlink for claude binary