Modernization of the legacy "assign test case version to multiple ACTIVE test plans" screen (lib/testcases/tcAssign2Tplan.php, gui/templates/dashio/testcases/tcAssign2Tplan.tpl) — GitHub issue #810.

The legacy Smarty popup is replaced by a standalone Dashio page (gui/templates/testcases/tcAssign2Tplan.html) backed by a plain-PHP REST BFF API (api/tcassign2tplan/index.php). The test case viewer toolbar Add to Test Plan button (openAdd2Tplan) now opens the modern screen instead of the legacy tcAssign2Tplan.php redirect (this was the last remaining HIGH-priority legacy redirect from issue #753, along with tcEdit.php — tcExport.php and tcCompareVersions.php were already converted).

Path: Test Spec → Test Case Viewer → toolbar Add to Test Plan URL: gui/templates/testcases/tcAssign2Tplan.html?tcase_id=..&tcversion_id=..&tproject_id=.. BFF API: GET /api/tcassign2tplan/?action=init + POST /api/tcassign2tplan/?action=add Rights: any authenticated session (parity with legacy — the standalone screen performs no extra menu-right check; the test case must belong to the given project context). Tracking issue: #810

The BFF reproduces the exact decision logic of tcAssign2Tplan.php for each ACTIVE test plan (status 1) of the project:

• It computes the linked test-case-version/platform pairs from testcase::get_linked_versions().
• For each plan it reads the plan's linked platforms (testplan::getPlatforms() → getLinkedToTestplanAsMap).
• A platform is rendered read-only (already linked) when the plan already has any version of this test case linked to that platform. Because TestLink does not allow different versions of one test case linked to the same plan, when the plan is already linked to a different version, the remaining platforms are hidden.
• When the plan is already linked to the same version, the remaining platforms are selectable.
• Add button shows only when at least one platform is selectable.

Section	Description
Heading	"Assign Test Case to Test Plan" + test-case identity (PREFIX-<id>:<name>)
Toolbar	Test Case name (+id +version) and locale switcher
Plan usage card	DataTables-style grid: checkbox · Version · Test Plan · Platform
Read-only rows	Already-linked pairs render a checked+disabled checkbox with an "(already linked)" note
Actions	Add (visible only when can_do) and Cancel

1. On load the page calls GET /api/tcassign2tplan/?action=init with tcase_id, tcversion_id, tproject_id. The BFF resolves the test case (validating it belongs to the project), its current version number + identity, and builds the plan/platform grid.
2. The user ticks the (version, platform) pairs to link for the desired plans.
3. Clicking Add issues POST /api/tcassign2tplan/?action=add with add2tplanid:{tplan_id:{platform_id:true}}. The BFF calls testplan::link_tcversions() per plan (audited, ASSIGN).
4. On success the grid reloads; the newly linked rows become read-only and the success banner shows "Added to N test plan(s)".

Case	Result
Missing tcase_id/tcversion_id	fatal banner ta2p.errNoContext
Test case not in project context	HTTP 404 Test case not found in project context
No test plans in project	"No test plans." message, no table
No plan selected on Add	client toast ta2p.warnSelect, no request
Empty add2tplanid on Add	HTTP 400 No test plan selected
Not authenticated	HTTP 401 {"status":"error","message":"Not authenticated"}

Method	Endpoint	Description
GET	/api/tcassign2tplan/?action=init	tc context + active plans grid with per-platform checkbox/linked state
POST	/api/tcassign2tplan/?action=add	link the TC version to the selected plan/platform pairs (link_tcversions)

Both routes require a valid session (401 otherwise).

• The BFF reuses the exact legacy grid logic from tcAssign2Tplan.php (linked-versions → per-plan target version → per-platform draw_checkbox/doAdd), so the read-only/already-linked behavior matches 1.9.20.
• Context validation walks nodes_hierarchy from the test case up to the project id, rejecting cases that do not belong to the requested project.

All labels, titles, headings, messages and placeholders use client-side TLi18n keys under the ta2p.* namespace (17 keys) present in all 10 locale bundles (gui/templates/i18n/{en,ro,de,es,fr,it,ja,pt,ru,zh}.json). Bundle consistency is enforced by tools/lint_i18n.py; every bundle passes python3 -m json.tool.

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TestLink 2.0.1 · Assign Test Case to Test Plan · Refs #810