[2.x] fix: register Input in the common export manifest

[imorland]

Changes proposed in this pull request:

Input is public extension API — documented attrs, shipped typings — but it was never listed in common.ts, the manifest that eagerly registers components with the export registry. It only reached the forum bundle incidentally, because AbstractGlobalSearch imported it statically.

#4909 replaced that <Input> with a plain button and dropped the import. That leaves the lazily-loaded SearchModal as the only forum-side importer, so webpack moved Input into the SearchModal chunk — in the built forum.js it now appears only as addChunkModule, never reg.add.

Extensions resolve core components synchronously via reg.get(), which returns undefined for a chunk-only module. So any extension rendering flarum/common/components/Input throws "The selector must be either a string or a component" on first render — fof/ui-kit's DiscussionSearch is one, which breaks fof/move-posts. It recovers only once something else loads the search modal chunk.

Adding Input to common.ts restores the eager registration.

Reviewers should focus on:

Whether other public components are in the same position. Bundle membership is an emergent property of which modules core happens to import statically, so any refactor that removes core's last eager import silently demotes a public component to chunk-only — no build error, no type error, just undefined at runtime in downstream extensions. A test asserting the documented public surface is reg.add'd in the built bundles would catch the next one; happy to follow up with that separately if wanted.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• The description above is written by me and describes what this pull request actually does.