Update docs to reflect frontend changes

[pauloiankoski]

Refreshes docs/subsystems/frontend.md and docs/architecture/conventions.md to match the current state of the React app after the welcome-screen work landed.

Notable updates:

• Provider stack now documents ReloadBannerProvider and the new five-provider nesting order.
• Adds a Routing section covering AppContent, the hasEverHadLicenseRef latch, and the implicit activation handoff to AppShell.
• Store shape reflects the new harborHosts slice, the expanded license slice (isRefreshing, refreshError), and the getHarborHostBasenames resolver.
• Documents the refreshLicense and refreshCatalog thunks, the enableFeature invalidation of host basenames, and the combined Refresh button behaviour.
• Selector list updated with new harbor-host, license, and legacy selectors.
• Adds a Hooks section listing useFeatureRow, useFilteredFeatures, useProductFeatureGroups, useWelcomeLicenseForm, and the resolvable-select pair.
• Component hierarchy reflects the new atoms/molecules/organisms/templates introduced for the welcome screen and reload banner.
• HarborData payload documents domain, version, and licenseKeyPrefix, plus the getHarborDataValue accessor pattern.
• Upgrade CTA section rewritten around buildUpgradeUrl (replacing the old buildChangePlanUrl flow).
• CSS scoping section adds the renameTailwindVars PostCSS plugin and notes the conventions rename from .lw-harbor to .lw-harbor-ui (with lw-harbor retained as the canonical prefix for IDs).
• Conventions identifier table updated: HarborError / harbor-error.ts, store name 'lw/harbor'.