[Docs]: Refresh the interface documentation and screenshots after the Studio redesign

[rnv812]

What is wrong

The Studio redesign (eventum-generator/eventum#179) rebuilt every screen, so the site now documents an interface that no longer exists. Every screenshot predates it, and the prose describes structures that were replaced, not merely restyled:

• studio/projects.mdx — documents the project page as three tabs (Input plugins, Event plugin, Output plugins) with Debugger / State / Editor sub-tabs. It shipped as a docked workspace: file explorer, tabbed code editor, stage inspector, and a console holding the debug tools.
• studio/instances.mdx — documents the instance page as one settings form plus a metrics modal. It shipped as Overview / Settings / Logs tabs with a live pipeline graph. The table gained Flow, Errors and Written columns, an All / Running / Inactive filter, record counts, and a distinct Finished state.
• studio/settings.mdx — covers Settings, Secrets and Management, and all three changed: Settings became a section rail with Save in the page header, Secrets gained an inline add form, Management became a console with identity cards, a live application log, and a danger zone.
• studio/monitoring.mdx — Performance / Flow stats / Charts / Instances sidebar became a dashboard: pipeline flow, throughput and failure charts, per-instance share of the output load, and resource tiles.
• studio/scenarios.mdx — the scenario page gained an aggregate status header with Start all / Stop all, and the template preview is inline instead of a modal.
• studio/overview.mdx, studio/index.mdx — the Home screen was rebuilt around recent projects, an instances rail and quick-nav tiles.
• core/introduction/features.mdx — shares /images/home/* with the landing page; all five pairs are pre-redesign.

What is needed

1. Rewrite the interface prose on every page above against the shipped UI. The editor feature list also omits find and replace (Ctrl/Cmd-F, rebuilt in [Feature]: Style the editor search panel in Studio eventum#196: match counter, case / regular-expression / whole-word toggles, replace on demand), Ctrl/Cmd-Alt-G go-to-line, and the file tree's size column with its 10 MB open limit.
2. Reshoot the 19 screenshot pairs in public/images/studio_ui/, light and dark.
3. Reshoot the 5 pairs in public/images/home/, used by the landing page (components/pages/home/Features/data/features.ts) and by core/introduction/features.mdx.
4. Sweep the rest of content/docs/ for interface vocabulary that moved — tutorials/csv-dataset.mdx carries one such reference.

The /images/studio_ui/ screenshots referenced from blog/eventum-2-4-0.mdx stay as they are: a release post is a record of that release.

Related, in another repo: .github/assets/studio_dark.png in eventum-generator/eventum is the README hero and needs the same reshoot.