feat(modlist): add updates-only filter to the mod list

[ModifAmorphic]

What changed

• New updates-only toolbar toggle on the Mods page: a session-transient filter that narrows the row list to rows flagged with an available update. It composes with the hide-disabled filter and the name search (AND), feeds the same no-matches empty state, never persists, and clears on an active-profile change.
• ModListViewModel.Reload now hydrates the persisted known-update flags before rebuilding the visible projection (the projection reads UpdateAvailable, and freshly built rows default it to false), and a landed update check reprojects, so the filter tracks live flag changes.
• New toolbar button after the hide-disabled toggle: Material "update" glyph, one stable shape with the selected class marking the active filter.
• Six new ModListFilterTests cases: the projection + Reload-ordering regression, AND-composition, live reprojection on a landed check, session-transient lifecycle (survives reload, clears on profile switch), no-matches/add-hint exclusivity, and the tooltip flip.
• Docs updated to match: AGENTS.md, docs/architecture/ui-architecture.md, docs/reference/ui.md.
• Nexus page description copy refresh (vanilla-install callout, Linux install/uninstall sections) and the ModRow_UpdateTooltipOpenFiles rewording ("Newer version available. Open the Nexus files page to download").

Why

The per-row update-action cell already carries the flagged-update signal, but with long lists the flagged rows scatter through the list. The toggle surfaces them at the list level in one click.