PhoenixKitProjects.
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The Whiteboards extension tab (Step 11): board list + canvas, rendered
by the projects hub via live_render with the extension-tab session
contract (see PhoenixKitHelloWorld.Web.ProjectHelloTabLive for the
contract reference). Mounts off-router only; no handle_params/3.
Drawing itself is core's MediaCanvasViewer LiveComponent over the
board's background file — annotation persistence, palettes, and tools
all live there; this LV only manages board rows and hands the component
a curated file map (Whiteboards.viewer_file/1).
Authorization: the hub renders extension tabs inside surfaces it has
already authorized (the admin show page / an authorized host embed), and
the tab session carries no scope — so, per the extension-tab contract,
this LV scopes every query by the session's project and does not re-run
Authz per event (same trust model as every contributed tab).
Board creation is additionally identity-gated: no current_user_uuid
in the session, no writes (the files table requires an owning user) —
and write-gated on the session's "can_write", which the HOST
resolves from its scope against this extension's declared write action
(final panel: mutations must not ride the view-only trust model).
Phoenix-first: the board list, create, and delete all work without JS;
the canvas needs core's Fresco/Etcher hooks (already shipped by
phoenix_kit.js) and renders the background image server-side either way.