fix: use URL keyboard for subscribe field

[sgallese]

Summary

Fixes #1257.

• Use URL keyboard options for the Subscribe dialog URL field.
• Preserve the existing Search IME action.
• Keep ClipboardTextField defaults unchanged for other callers.

Test plan

• Added unit tests for the URL keyboard options and IME-action merge.
• Tested https://alignedtrack432.neocities.org/feed.xml; it keeps the URL keyboard active and resolves to the Sidetracked subscribe candidate.
• Tested a duplicate feed URL; it keeps the URL keyboard active and shows the existing Already subscribed error.

Evidence

• Recording: issue URL resolves to the Sidetracked feed
• Recording: valid feed search reaches subscribe options
• Recording: duplicate feed shows Already subscribed