OR-280 Nudge new installs to run the demo experiment

[myles332]

Summary

Sep 9–12 install analytics showed the largest funnel leak between "onboarding complete" and "first message sent" (43.8%). The demo welcome pushed users to read the recorded sessions rather than send anything. This PR makes the first message the obvious next step.

• Welcome modal primary button is now Try the demo experiment; it hands focus to the composer, which already holds the run-experiment prompt. The telemetry choice value stays explore_demo.
• The prefilled prompt stays on offer until the user has sent anything in the demo. A send inside a recorded session appends to it rather than creating a new one, so the check compares each session's activeLeafId against its seeded leaf (DEMO_SEEDED_LEAF_IDS) instead of only looking at session ids.
• A dismissible hint above the composer ("This prompt runs a small 200-step training probe on your machine. Hit enter to get started.") with a ring on the send button while the prefill is untouched. Textarea is linked via aria-describedby; dismiss returns focus to the composer.
• Once a demo run is running, a second dismissible hint appears: "The experiment is now running. Tap [Logs] to monitor it." The Logs button opens that run's logs. It seeds from live run events and from the runs baseline, so it survives a reload mid-run, clears when the run leaves the running state, and is announced via a polite live region.
• First demo visit opens only the experiments tab in the right pane (the overview artifact tab is no longer pre-opened).
• Default right-pane width for installs with no saved layout is now a quarter of the window, capped at 480px (was 40%/760px). The projects home used to write the old 760 into the saved layout before the demo opened, so the default now lives in ui/src/panelLayout.ts and both callers use it. Existing installs keep their saved width.
• Locale catalogs (en, zh-CN, fa, ar) updated, including the Arabic locale that landed on main mid-review; tour_explore_demo renamed to tour_run_demo_experiment everywhere. ui/dist rebuilt on the merged tree.

Not changed: the demoOverviewLeading logic in App.tsx that pins the overview artifact first when present. Nothing opens that tab automatically anymore, so it only applies to layouts saved by older builds or a manually opened overview. Left for a separate cleanup.

Test plan

• node ui/scripts/check-i18n.mjs, node ui/scripts/check-styles.mjs, UI typecheck, ui/tests (162 pass)
• cargo fmt --check, cargo clippy --all-targets -D warnings, cargo build --locked, cargo test --locked (830 pass) on the merged tree
• Fresh dev slot: onboarding → demo lands with hint, prefill, and experiments-only pane; sending the prompt withdraws the prefill; the running hint appears once the probe starts and its Logs button opens the run
• Fresh install: confirm the right pane opens at ~25% width on the first demo landing (not 760px)
• Dismiss either hint with keyboard only; focus should land in the composer
• Reload the page while the probe is running: the running hint should return
• Leave the demo and return after sending: the prompt hint must not reappear
• Dark theme: both hint bubbles and the send-button ring legible
• Arabic or Persian locale: the "Tap [Logs] to monitor it" sentence reads correctly right-to-left

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