Docs: remaining integration-leaf screenshots still show empty states (need example data / sources)

[rubenvdlinde]

Follow-up to the screenshot refresh (#1911 / #1912). Most leaves now show real linked data; these still show their restyled empty state and need backing data or config to populate:

• email — needs a configured Mail account + messages (SMTP disabled in dev).
• xwiki / openproject — need an external OpenConnector source (no external service in dev). See Investigate adopting native NC apps (xwiki, integration_openproject) as the connection for external leaves #1908 for the native-app alternative.
• activity — OR object operations don't emit NC Activity events, so the feed is empty. Either emit activity events from OR object CRUD, or seed oc_activity rows for the demo object.
• shares — ShareLinksController::create / ShareLinkService rejects a plain folder file ("File does not belong to this object"); seeding requires attaching a file through OR's own files flow first. Document or provide a seed path.
• time-tracker — blocked by the link-lookup bug (separate issue).

Minor copy

• CnFlowTab header button still reads "Link existing automation" — the tab was renamed to Flow (nextcloud-vue fix #437); align the button/empty copy to "Flow" too.

[rubenvdlinde]

Additional finding while refreshing screenshots: a backend-registered sync-contract provider (tab label "Synced from") now appears in the widget tab strip with the generic help-circle fallback icon — its declared icon isn't in the CnIcon registry used by the tabbed widget. It's not in nextcloud-vue's builtin/leaves set (so the icon parity test doesn't cover it); the icon needs registering wherever sync-contract is registered (OR backend provider → frontend), or added to nextcloud-vue's integration icon set if it should be a builtin.

• Register the sync-contract ("Synced from") icon so it renders a real glyph instead of help-circle.