Fix inconsistent disabled styling on logged-out repo header buttons (#37406)

Make the watch, star, and fork buttons in the repo header consistent for
logged-out users:

- Apply the same look to all three buttons (number labels
included), instead of only the action button being grayed.
- Clicking any of them while logged out now leads to the login page
(with a redirect back) instead of being inert.
- Split the per-button markup out of `header.tmpl` into a dedicated
`templates/repo/header/` folder (`fork.tmpl`, `star.tmpl`,
`watch.tmpl`).

---------

Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/header/watch.tmpl

@@ -0,0 +1,19 @@
+<div class="ui labeled button" {{if not $.IsSigned}}data-tooltip-content="{{ctx.Locale.Tr "repo.watch_guest_user"}}"{{end}}>
+	{{$buttonText := ctx.Locale.Tr "repo.watch"}}
+	{{if $.IsWatchingRepo}}{{$buttonText = ctx.Locale.Tr "repo.unwatch"}}{{end}}
+	<a role="button" class="ui compact small basic button" aria-label="{{$buttonText}}"
+		{{if $.IsSigned}}
+			data-fetch-method="post"
+			data-fetch-url="{{$.RepoLink}}/action/{{if $.IsWatchingRepo}}unwatch{{else}}watch{{end}}"
+			data-fetch-sync="$closest(.ui.labeled.button)"
+		{{else}}
+			href="{{AppSubUrl}}/user/login"
+		{{end}}
+	>
+		{{svg "octicon-eye"}}
+		<span class="not-mobile" aria-hidden="true">{{$buttonText}}</span>
+	</a>
+	<a class="ui basic label" href="{{$.RepoLink}}/watchers">
+		{{CountFmt .Repository.NumWatches}}
+	</a>
+</div>