Feature: Custom Dashboard Builder (Configurable Widgets & Layouts)

## Problem
Different users need different observability views (ops-focused, analytics-focused, debugging-focused). The current dashboard layout is fixed.

## Proposal
Introduce a **Custom Dashboard Builder** where users can assemble widgets and save named layouts.

## Scope
- Widget catalog (stat cards, active agents, event feed, heatmap, token chart, custom query tile)
- Drag-and-drop grid layout with resize handles
- Multiple saved layouts (e.g., "Incident", "Daily Monitoring")
- Per-layout refresh strategy and data source settings

## UX Requirements
- One-click switch between saved layouts
- Reset to default layout
- Unsaved-change warning when editing layout

## Technical Notes
- Persist user layout JSON in SQLite (`dashboard_layouts`)
- Use responsive grid that preserves desktop/mobile behavior
- Build widget interface contract (`id`, `config schema`, `render`, `data requirements`)

## Acceptance Criteria
- [ ] User can create, rename, duplicate, and delete layouts
- [ ] User can add/remove/reorder widgets and resize them
- [ ] Layout state persists across restarts
- [ ] Default layout remains available as fallback
- [ ] Existing dashboard data APIs can back widgets without regressions