Strengthen CI and test coverage to catch regressions earlier and update README

[ndoschek]

ci: schedule nightly after Theia next publish, build apps, guard against run-loop helpers

• Move the scheduled run to weekday mornings (0 4 * * 1-5), a few hours
  after Theia publishes its 'next' version at midnight UTC, so the run
  validates the freshly released 'next' instead of the previous day's.
• Make generate-and-build-all-templates.sh actually bundle the generated
  browser and electron apps ('npm install' compiles the extension but not
  the apps), so build-time regressions (e.g. dependency hoisting) are caught.
• Add a fast unit test that fails if any generator helper method is public,
  since yeoman auto-runs such methods as run-loop tasks with the wrong
  arguments; and assert the resolved electron version is concrete.

test: add regression tests mapped to recent fixes

• Add a tree-widget generation test (previously the only untested template).
• Assert '@types/react' is present in the widget and tree-widget
  devDependencies (guards the TSX compile fix).
• Assert the generated root package.json has exactly one 'test' script
  (guards the duplicate-key fix, which JSON.parse would hide).

docs: update README

• Point the logo and build badge at the current 'eclipse-theia' org; the
  repo moved from 'theia-ide', so the badge showed no status.
• Drop the stale 'yo (version 4.x.x)' hint; any current yo version works.