Refactor developer dashboard widgets

[roncodes]

Summary

• Replace the static developer console home dashboard with the shared Dashboard component using a sticky header.
• Register a dedicated developers dashboard with 14 redesigned widgets: 8 KPI tiles and 6 operational/resource widgets.
• Add developer-focused widgets for API traffic, error rate, latency, webhook success, active credentials, active webhook endpoints, endpoint health, event stream, recent activity, and quick resources.
• Add Webhook Failures and Events Emitted KPI tiles so KPI widgets render as two complete rows in the 12-column dashboard grid.
• Normalize default widget sizing so non-KPI widgets render in balanced two-column rows without container overflow.
• Keep the previous API metrics widget available as a legacy widget with default: false.
• Add responsive dashboard/widget styling so widget chrome stays fixed, widget content does not overflow, and scrolling is contained inside widget bodies.
• Add integration coverage for the generic KPI tile and quick resources widget.

Companion PR

This dashboard consumes new internal aggregate metrics endpoints from the companion core-api PR:

• Refactor internal metrics API endpoints core-api#221
• Widgets call the internal metrics/dev/* endpoints.

Validation

• ./node_modules/.bin/eslint . --cache
• ./node_modules/.bin/ember-template-lint .
• ./node_modules/.bin/stylelint "**/*.css"
• ./node_modules/.bin/fleetbase-intl-lint
• PATH=/Users/ron/.nvm/versions/node/v22.22.2/bin:$PATH ./node_modules/.bin/ember build --environment=production
• ./node_modules/.bin/ember build --environment=production under the default Node 24.6.0 fails during shared CSS minification with broccoli-persistent-filter:CleanCSSFilter / util.isRegExp is not a function; the same build passes on Node 22.
• ./node_modules/.bin/ember test was attempted, but the runner fails before app/tests load on an existing vendor runtime error in ember-engines/components/link-to-external: Class extends value [object Object] is not a constructor or null.

Documentation and API Reference Impact

• This is a developer console dashboard UI change.
• No public API behavior changes are introduced in this package.
• No fleetbase/postman update is required for this PR.