[gnacho]

This adds rtlplayground as a vendor in the integrations catalog, next to asuswrt, bbox, routeros and unifi. Picking it in the create form shows the flashing note inline: Realtek switches can run rtlplayground (the RTLPlayground fork) to become NetPulse agents, with links to both flashing guides in the docs.

Two things worth stating plainly:

• The seed entry uses mode local with arches ["mipsle"], tagPrefix rtlplayground/v, binary rtlplayground-pusher and target rtlplayground/cmd/rtlplayground-pusher. Those binary and target paths are placeholders: today the RTL837x switches arrive as agents through the beacon and console flow (see docs/AGENTE-RTL-SWITCH.md), not through a pusher I built. The catalog entry makes the vendor selectable and the note contextual; wiring an actual pusher binary would be its own piece of work.
• The catalog tests asserted four seed vendors; they now assert five and check the rtlplayground row (asset derives from the vendor id, matching the routeros/unifi/asuswrt pattern).

It also removes the flashing note from the External agents dialog in Settings: the note now lives where it belongs, right where the vendor gets picked.

Refs #1782, refs #1784.