Replace manual resource ID entry with a searchable, filterable resource
picker that uses live WebSocket state. Support selecting multiple
resources (up to 50) for combined fleet reports.
Multi-resource PDFs include a cover page, fleet summary table with
aggregate health status, and condensed per-resource detail pages with
overlaid CPU/memory charts. Multi-resource CSVs include a summary
section followed by interleaved time-series data with resource columns.
New POST /api/admin/reports/generate-multi endpoint handles multi-resource
requests while the existing single-resource GET endpoint remains unchanged.
Also fixes resource ID validation regex to allow colons used in
VM/container IDs (e.g., "instance:node:vmid").
