Preserve web session list visibility on resize

[edwin-zvs]

Summary

• track current session-list visibility separately from responsive CSS classes
• preserve the current hidden state across keyboard/window resize events instead of re-reading stale localStorage
• add web smoke coverage for narrow-layout resize after auto-hiding the list

Investigation

On narrow screens, selecting a session auto-hides the list with persist=false so reload/desktop preference is not changed. The resize handler later re-read localStorage on every resize; if storage still said visible, a virtual-keyboard resize or terminal button focus path could expand the list even though the user did not press the menu button.

Validation

• node script parse check for the inline web UI script
• cargo build -p agentd
• cargo test -p agentd-e2e web_client_loads_and_websocket_connects -- --nocapture
• cargo build
• git diff --check

Built debug binaries

• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agent
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-mcp
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-adapter-antigravity
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-adapter-claude
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-adapter-codex
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-adapter-shell
• /Users/moon/agentd/.claude/worktrees/webui-session-list-resize/target/debug/agentd-adapter-zarvis

Relevant binary: this PR changes daemon web UI assets, so run agentd to test the web client behavior.