Tests: unit coverage plus the unresolvable-reference e2e cases

[romain-pm]

Part of #749.

• Vitest over getLinkProps and resolveContentLink: falsy target, "" and whitespace-only string, string target, scheme allow-list — accept http https mailto tel ftp / #, reject javascript: data: blob: vbscript: — target validation and omission, rel on _blank, isCurrent vs isAncestor (including /home/news vs /home/newsletter, and two distinct proxies for the same node, which must compare equal), isCurrent override, label fallback, parameters + hash ordering on node and string targets, missing translation.
• Unit test asserting no LinkState key (navigable, isCurrent, isAncestor, label) reaches the DOM.
• Unit test asserting the label agrees with getDisplayableName() for jnt:page, nt:file and an untitled node.
• Cypress: publish a link whose target is not published, assert the page renders the fallback and no <!-- Module error appears. No test exercises an unresolvable reference today — getNodePropsTest.cy.ts:109-111 only asserts the happy path for a weakreference.
• Cypress: rename the target of a rendered link, assert the cached fragment is flushed.
• Cypress: publish a previously-dangling target and assert the fallback fragment is flushed — this is what the { uuid } dependency form buys, and { node } cannot.
• Cypress: render the same nav on two pages via an AbsoluteArea and assert aria-current follows the page (the cache.mainResource case).
• Page-builder case: a <JLink> in the builder must emit an /cms/editframe/… href, so a click navigates the builder instead of opening a new tab. Assert on the delivered DOM, not the component output — EditModeFilter rewrites both the href and target.

Unit coverage landed in #751 (105 cases); the Cypress cases still need a running instance.