fix: preserve tag filter after toggling assignment status

[Gasg10]

Fixes #89

Problem

When a tag filter was applied in the Current Assignments page and the teacher toggled the Active? checkbox, the tag filter was lost after the redirect. The page returned to the default state showing all assignments.

Root Cause

The toggleAssignmentStatus controller did not receive or preserve the active tag filter. After toggling, it always redirected to /assignment/my without any query parameters, discarding the selected tags.

Fix

Three changes were made:

1. AssignmentController.kt — Added tags as a @RequestParam to toggleAssignmentStatus and used it in the redirect:

fun toggleAssignmentStatus(@PathVariable assignmentId: String,
                           @RequestParam(name = "tags", required = false) tags: String?,
                           ...): String {
    ...
    return if (tags != null) "redirect:/assignment/my?tags=$tags" else "redirect:/assignment/my"
}

2. AssignmentController.kt — Added currentTags to the model in listMyFilteredAssignments:

model["currentTags"] = tags ?: ""

3. teacher-assignments-list.html — Added a hidden input to pass the current tags when the toggle form is submitted:

<form th:action="@{'/assignment/toggle-status/' + ${assignment.id}}" method="post">
    <input type="hidden" name="tags" th:value="${currentTags}"/>
    ...
</form>

Testing

1. Start the application locally
2. Log in as teacher
3. Navigate to Assignments → Current Assignments
4. Select a tag filter (e.g. kotlin) — only filtered assignments are shown
5. Toggle the Active? checkbox of an assignment
6. The tag filter is now preserved after the redirect