Feature/implement customizable panel

[pikann]

Summary

Replaces the old fixed "overview" pages (ProjectOverviewSection, InstanceOverviewSection, DashboardPage, widgets.tsx) with a customizable, panel-based dashboard builder — drag-and-drop charts, tables, and text blocks backed by user-authored SQL, across three scopes:

• Project dashboard — one per project
• Admin dashboard — instance-wide, requires users.write
• Integration dashboards — one per host Backlog/Sprint/Timeline "Dashboard"-type view

Backend

• panels.go, views.go, types.go — new panel/view CRUD and layout endpoints, plus panel data fetching with a TTL cache (5 min) and force-refresh (panels_cache_test.go).
• query_guard.go — safety layer for user-authored panel queries: single SELECT/WITH statement only, blocks mutating statements and introspection (information_schema, pg_catalog, dblink, lo_*, etc.), requires the {{project_id}} placeholder on project/integration-scoped queries (substituted server-side as a bound parameter to prevent cross-project data leakage), and auto-appends LIMIT 500 when absent. Replaces the previous per-table column whitelist — sensitive-column redaction is now handled centrally by the host's db_query/db_query2 runtime instead of being this plugin's responsibility.
• New migrations for dashboard/panel/view tables, including a singleton-per-host-view constraint.
• overview.go removed; superseded by panels.go/views.go.

Frontend

• PanelGrid.tsx, PanelCard.tsx, PanelEditor.tsx, DashboardBody.tsx, SimpleCharts.tsx — new drag-and-drop grid layout, panel rendering (chart/table/text), and query editor.
• DashboardIntegrationView.tsx — new host-view integration entry point.
• New components/ui/{button,chart,dialog}.tsx, lib/{i18n,utils}.ts, and use-sync-external-store shims ported in to keep the plugin self-contained and avoid dual-React-instance issues.
• presets.ts — library of ready-made SQL query templates (status breakdown, burndown/velocity, overdue tasks, etc.).
• shared.tsx / widgets.tsx removed; superseded by the panel components above.

MCP / Skill

• mcp/src/index.ts — new dashboard_* tools for view/panel CRUD, layout updates, query preview, and panel data retrieval (project, admin, and integration scopes).
• skills/paca-dashboard-builder/SKILL.md — new skill documenting the end-to-end workflow (resolve view → write/preview query → create panel → verify data), the query safety rules, a schema reference, and copy-paste preset queries.

Other

• plugin.json — new routes/permissions for the added views and panel APIs.
• README.md — updated to describe the new dashboard builder.

Test plan

• go test ./... in backend/ (includes new panels_cache_test.go and expanded plugin_test.go)
• Create a project dashboard, add chart/table/text panels using preset and custom queries, confirm layout persists
• Verify query guard rejects mutating/introspection queries and enforces {{project_id}} scoping
• Create an admin dashboard panel as a non-admin and confirm it's rejected
• Create a Backlog/Sprint/Timeline "Dashboard" host view and confirm the integration dashboard resolves and renders panels
• Confirm cached panel data respects TTL and refresh: true bypasses it