Edit-mode replace affordance for broken singular linksTo

[habdelra]

What

In edit mode, a broken singular linksTo now offers a replace affordance: a "Link [type]" button rendered alongside the broken-link placeholder and the remove control. Clicking it opens the card chooser and swaps the broken reference for a working one directly, without first removing the link to fall back to the empty state.

The broken state stays visually distinct from the never-set state:

• Never set — the bare "Link [type]" button only.
• Broken (error / not-found) — the placeholder with the URL surfaced, a remove (X) control, and the "Link [type]" replace control.

Both write controls are hidden when the editor is read-only; the placeholder and its URL still render so the broken target stays visible.

Preview

Singular linksTo (with the new replace button) and plural linksToMany (broken slot keeps its drag handle + remove among present cards) in edit mode:

Where

• packages/base/links-to-editor.gts — the broken branch of LinksToEditor gains the replace button, stacked with remove in the trailing controls column.

Tests

packages/host/tests/integration/components/linksto-broken-link-placeholder-test.gts:

• broken edit view shows the placeholder, URL, remove, and the labelled replace control;
• removing reverts the slot to the empty "Link" affordance;
• a read-only broken link shows the placeholder with neither control.

Plural linksToMany edit-mode handling (per-slot placeholder + remove) is carried by #4983, which this PR is stacked on.

CS-11227

[Copilot]

Pull request overview

Adds a “replace” affordance for broken singular linksTo fields in edit mode, allowing users to swap a broken reference for a valid one via the card chooser without first clearing the field. This aligns singular linksTo broken-state UX more closely with the existing plural handling and clarifies the distinction between “never set” vs “broken”.

Changes:

• Render a “Link [type]” replace button alongside the broken-link placeholder and remove control (writeable only).
• Stack broken-state controls in the trailing column next to the placeholder.
• Expand integration coverage to assert broken edit-mode controls, remove→empty transition, and read-only behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/base/links-to-editor.gts	Adds replace button to broken-link edit state and stacks remove/replace controls; includes styling.
packages/host/tests/integration/components/linksto-broken-link-placeholder-test.gts	Adds/updates tests verifying broken edit UI shows placeholder+URL+remove+replace, removal returns to empty “Link” state, and read-only hides controls.

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[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files      1 suites   1h 47m 53s ⏱️
2 815 tests 2 800 ✅ 15 💤 0 ❌
2 834 runs  2 819 ✅ 15 💤 0 ❌

Results for commit db32d6c.

Realm Server Test Results

    1 files      1 suites   10m 58s ⏱️
1 502 tests 1 501 ✅ 0 💤 1 ❌
3 186 runs  3 185 ✅ 0 💤 1 ❌

Results for commit db32d6c.

For more details on these errors, see this check.

[lukemelia]

It would be good to review this with design team after it lands.