Resolves #7364(FR-2870)

Summary

When the manager's vfolder host info fetch (/folders/_/hosts) fails (storage proxy 500, manager unreachable, etc.), the Session Launcher previously surfaced a generic page-level error from BAIErrorBoundary ("An error has occurred.") with a reload button — the user could not tell what was actually wrong and reloading didn't help.

This PR replaces that experience with a dedicated, recognizable error UI at the exact existing fetch site:

• useProjectResourceGroups (packages/backend.ai-ui/src/hooks/useProjectResourceGroups.ts) is the live host-fetch path on the Session Launcher page — it powers BAIProjectResourceGroupSelect, which the launcher renders inside ResourceAllocationFormItems. It already runs /scaling-groups and /folders/_/hosts in parallel and the useSuspenseTanQuery underneath throws on any failure; before this change both failures were indistinguishable.
• Switch the parallel fetch from Promise.all to Promise.allSettled, then discriminate: a host-info failure is re-thrown as a tagged StorageHostFetchError, while a scaling-groups failure is re-thrown as-is and bubbles to the generic boundary. Host-info failure takes precedence when both fail because SFTP filtering depends on it.
• Wrap the /session/start route with a new StorageHostFetchErrorBoundary whose fallback matches the BAIErrorBoundary look (<Result status="warning">) but shows the message "Failed to fetch storage host information." (KO: "스토리지 호스트 정보를 가져오는데 실패했습니다.") and a "Go back to the previous page" primary button that calls history.back(). Any non-StorageHostFetchError is re-thrown so the outer BAIErrorBoundary continues to render its generic UI for unrelated failures.

The useCurrentProject atom's Promise.allSettled path (which silently swallows host failure into vhostInfo = undefined) is intentionally out of scope for this PR.

Implementation

• packages/backend.ai-ui/src/hooks/useProjectResourceGroups.ts: introduce StorageHostFetchError extends Error; rewrite the queryFn to await Promise.allSettled, throw StorageHostFetchError(hostsResult.reason) when the host fetch rejects, re-throw the scaling-groups rejection otherwise. The success return shape is unchanged.
• packages/backend.ai-ui/src/hooks/index.ts: export the new StorageHostFetchError class alongside the existing useProjectResourceGroups export.
• react/src/components/StorageHostFetchErrorBoundary.tsx (new): wraps children with react-error-boundary's ErrorBoundary. The fallback imports StorageHostFetchError from backend.ai-ui, checks error instanceof StorageHostFetchError, and either renders the dedicated Result UI or re-throws so the outer BAIErrorBoundary handles unrelated errors.
• react/src/routes.tsx: wraps <SessionLauncherPage /> at the /session/start route with StorageHostFetchErrorBoundary so the dedicated UI replaces the page body while the breadcrumb and outer layout remain intact.
• resources/i18n/en.json, resources/i18n/ko.json: add errorBoundary.StorageHostFetchFailedTitle and button.GoBackToPreviousPage. Other locales fall back to English (fallbackLng: 'en'); /fw:i18n can fill them in.

How to test

1. Force the storage host fetch to fail (e.g. kill the storage proxy or break the manager's /folders/_/hosts route).
2. Open the Session Launcher at /session/start.
3. Expected: the page body renders a warning Result with the title "Failed to fetch storage host information." and a "Go back to the previous page" button. The breadcrumb above remains visible.
4. Click the button — the browser navigates back via history.back().
5. Trigger an unrelated error elsewhere on the launcher (or break only /scaling-groups) — the existing BAIErrorBoundary behavior (page reload prompt, debug info under globalThis.backendaiwebui.debug) is unchanged.

Verification

bash scripts/verify.sh:

--- Relay: PASS ---
--- Lint: PASS ---
--- Format: PASS ---
--- TypeScript: FAIL ---  (pre-existing, unrelated files only — identical on main)

The TypeScript failures are pre-existing errors on main (StatisticsPage.tsx, StorageHostSettingPage.tsx, UserCredentialsPage.tsx, UserSettingsPage.tsx, VFolderNodeListPage.tsx, plus the pre-existing ErrorBoundary JSX type error at SessionLauncherPage.tsx:1433) — none introduced by this change.

Follow-ups

• Translate the new i18n keys for the remaining 19 locales via /fw:i18n.
• Address the silently-swallowed host failure in useCurrentProject's resourceGroupsForCurrentProjectAtom (Promise.allSettled → vhostInfo = undefined masks the failure and produces an empty nonSftpResourceGroups). Deliberately out of scope for this PR.