fix(issue): Replace stopwatch toggle with explicit start/stop actions (#34818)

This PR fixes a state de-synchronization bug with the issue stopwatch,
it resolves the issue by replacing the ambiguous `/toggle` endpoint
with two explicit endpoints: `/start` and `/stop`.

- The "Start timer" button now exclusively calls the `/start` endpoint.
- The "Stop timer" button now exclusively calls the `/stop` endpoint.

This ensures the user's intent is clearly communicated to the server,
eliminating the state inconsistency and fixing the bug.

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/base/head_navbar.tmpl

@@ -197,7 +197,7 @@
 					<span class="stopwatch-issue">{{$activeStopwatch.RepoSlug}}#{{$activeStopwatch.IssueIndex}}</span>
 				</a>
 				<div class="tw-flex tw-gap-1">
-					<form class="stopwatch-commit form-fetch-action" method="post" action="{{$activeStopwatch.IssueLink}}/times/stopwatch/toggle">
+					<form class="stopwatch-commit form-fetch-action" method="post" action="{{$activeStopwatch.IssueLink}}/times/stopwatch/stop">
 						{{.CsrfTokenHtml}}
 						<button
 							type="submit"