Dashboard: sidebar navigation + overview page

[matt-bedda]

Overview

Replace the current tab-bar navigation with a collapsible sidebar (like OpenClaw's dashboard). Add an Overview page as the default landing page.

Sidebar Navigation

• Collapsible sidebar (220px expanded, icon-only collapsed)
• Groups: Control (Overview, Cron, Tasks, Pipeline), Agents, Settings (Config, Logs)
• Active state highlighting, group collapse/expand
• Mobile: horizontal nav bar instead of sidebar

Overview Page

Dashboard landing page showing:

• Connection status (green dot + uptime)
• System health (memory, disk, CPU)
• Key metrics cards: active agents, cron jobs, tasks ready, total submissions 24h
• Recent activity feed (last 10 cron runs with status)
• Cost summary (24h API spend)

Data Sources

• GET /health for uptime
• GET /api/cron/jobs for job count + recent runs
• GET /api/agents for active agent count
• GET /api/tasks (once implemented) for task count
• New: GET /api/stats endpoint returning aggregated metrics from cron_runs table

Priority: P2