Fresh installs no longer start a full library scan (with per-file SHA256
and sidecar writes) automatically at startup. Instead a welcome page
lists the configured model roots, estimates the scan cost from a quick
hash-free walk, and lets the user pick the sidecar storage layout and
UI language before confirming. Deferring shows an empty library; the
existing refresh button starts the scan later. Upgrading installs are
detected via a non-empty persisted cache snapshot (row-count check, so
the empty file created by scanner construction no longer masquerades
as an existing library) and keep the old behavior.
Bundled fixes surfaced by the new flow:
- broadcast an init-progress error when the startup scan fails instead
of silently landing on an empty library; the initialization page now
shows the error with a retry button
- skip the onboarding language modal/tutorial while the welcome or
initialization page is up (its spotlight targets don't exist there,
so the tour was silently auto-completed and never shown)
- promotional banners wait until onboarding is completed or skipped;
alert banners (startup messages, cache health) are unaffected
- honor the hidden attribute in first-run/initialization CSS so the
no-roots hint and error boxes don't render as empty frames
