docs(studio): rewrite the interface docs and reshoot every screenshot

[rnv812]

What this does

Rewrites the interface documentation against the Studio that shipped with the redesign, and replaces every screenshot with one taken from the current build.

Closes #52.

Prose

Every page under content/docs/studio/ described a structure that was replaced, not restyled. Each is rewritten against the shipped interface:

• Home (overview.mdx) — retitled from "Overview"; the screen is now recent projects, an instance-state summary with recent instances, and quick-nav tiles. The page keeps its overview slug so existing links, including the ones in the changelog, still resolve.
• Monitoring — the four-gauge / flow-stats / charts / sidebar layout is now a dashboard: pipeline flow, throughput, a failures chart that appears only while something is failing, per-instance share of the output load, and resource tiles.
• Projects — the three-tab editor is now a docked workspace. Documents the stage switcher, the Explorer, the Editor, the Inspector and the per-stage Console, plus the state the workspace falls back to when generator.yml cannot be loaded.
• Instances — documents the Flow, Errors and Written columns, the All / Active / Inactive filter, the record counts, the four resting states (Active, Idle, Finished, Failed) and the two transient ones, the per-row menu, and the instance page with its Overview / Settings / Logs tabs.
• Scenarios — the aggregate status header with Start all / Stop all, and the template preview now being inline rather than a dialog.
• Settings and management — Settings became a section rail with Save in the page header, Secrets gained an inline add form, and Management became a console with identity cards, a live application log and a danger zone.
• Studio index — the area table and cards follow the sidebar's own grouping; meta.json matches that order.

Gaps the issue did not list, now covered: find and replace with its match counter and case / regular-expression / whole-word toggles, Ctrl/Cmd+Alt+G go-to-line, the file tree's size column and the 10 MB open limit, instance rename and clone and which states allow them, the Inspector's generated-YAML view, console collapse and maximize, the unsaved-changes badge and prompt, per-section dirty markers in Settings, scenario membership from the instance page, and the queue size approximation.

features.mdx keeps its five images and its Studio section now names what Studio actually does.

Screenshots

All 24 pairs in public/images/studio_ui/ and all 5 pairs in public/images/home/ are new, 1920×1080, light and dark, matching the resolution the previous set used.

They were taken from a purpose-built workspace rather than an empty install, so the interface is documented carrying real data: 18 projects, 16 instances across every state, twelve generators running at ~120 events/s in total, live write failures on one instance and produce failures on another, four scenarios, and a four-generator scenario exchanging four keys of global state so the data-flow diagram has real edges in both directions.

Each screen was composed for what it is about — the timestamp preview shows three overlaid time patterns across three days, the formatter preview shows a real batch and its result, find and replace sits on a positioned match with the replace row open, and the metrics dialog is opened on the instance whose writes are failing.

Release post

blog/eventum-2-4-0.mdx is a record of that release, so its five screenshot pairs move to public/images/blog/eventum-2-4-0/ unchanged and the post's paths follow them. Only the paths in that file change.

Sweep

faq.mdx and tutorials/csv-dataset.mdx referred to the dashboard as the Overview page and to the State view as a tab; both now use the shipped vocabulary. Changelog entries are left as written.

Verification

pnpm build passes. Every /images/** reference in content/ and components/ resolves to a file on disk, and no image on disk is unreferenced.