feat(pools): import an existing pool from the web UI

[gnacho]

Problem

The import operation already existed in the backend (POST /api/pools/import), but the web UI only listed importable pools as plain text and never imported anything. The Pools button called importPool() with no name, and the missing-pools banner only navigated to Pools. After a power loss where a pool fails to auto-import, there was no self-service way to bring it back up from the UI.

Changes

• Add an ImportPoolModal: lists importable pools (zpool import), lets the admin select one, requires typing the pool name to confirm, imports it (zpool import <name>), then refreshes and toasts.
• Wire the Pools "Import existing pool" button to the modal (previously it only printed the list and never imported).
• Make the Dashboard missing-pools banner actionable: an "Import" button per missing pool (admin only) opens the modal preselected.
• Fix HttpProvider.importPool(name): the backend answers 202 with no body, so reading r.importable threw on an undefined response.
• ES/EN i18n strings.

No backend change is needed: PoolImportList / PoolImport and the endpoint were already in place.

Verification

• go test ./internal/... green.
• npm run build OK.
• Playwright E2E against the frontend mock provider (demo mode): 9/9 (button -> modal -> lists importable -> select -> confirm -> import -> toast -> modal closed, 0 page errors).
• Preview build deployed to a test instance: served bundle contains the modal and POST /api/pools/import responds {"importable":[]} (HTTP 200) with an admin session.

Closes #138