Modernized DataTables parity for "Assign Test Case to Test Plan" (gui/templates/testcases/tcAssign2Tplan.html).

The plan grid was a plain HTML table — no sort, search, pagination, or per-page length menu (a regression vs legacy dashio/testcases/tcAssign2Tplan.tpl which used DataTables.inc.tpl with DataTablesSelector="#item_view" and length menu [[25,50,75,-1],[25,50,75,"All"]]).

• DataTables 1.13.7 (Bootstrap 5 theme, CDN) included: CSS dataTables.bootstrap5.min.css, JS jquery.dataTables.min.js + dataTables.bootstrap5.min.js (same CDN pattern as suiteView/tcScripts).
• renderPlans() re-initializes #planTable with:
  • pageLength: 25, lengthChange: true, length menu [25,50,75,All]
  • searching: true, search box (localized via common.search)
  • orderable columns, checkbox column non-orderable
  • destroys any prior instance (DataTable(...).destroy()) before re-init, avoiding "cannot reinitialise DataTable" on repeated adds/re-renders.

• Legacy/modern diff measured during INVESTIGATION (issue comment).
• JS+i18n statically validated (syntax check, all bundle keys present).
• Browser round-trip requires a seeded testproject/testplan/testcase — the CI-injected DB is a fresh import with no such fixture yet, so full UI interaction scripting is pending a fixture (tmp/TLU_Test_Cases.md lists the manual steps).

---

Restored the legacy quick-navigation icons next to the test-case identity in the toolbar (legacy dashio/testcases/tcAssign2Tplan.tpl:47-51 dropped them during modernization).

Toolbar showed only Test Case: <name> as plain text — no way to jump to the Execution History or the Test Case Viewer while the assign popup is open.

gui/templates/testcases/tcAssign2Tplan.html toolbar renders two icon links right after the test-case identity (hidden until the screen loads):

• clock icon (title ta2p.execHistory) → openHistory() → window.open('/gui/templates/execute/execHistory.html?tcase_id=..&tproject_id=..', 'execHistory_<id>', 800x600).
• pencil icon (title ta2p.design) → openDesign() → window.open('/gui/templates/testcases/tcView.html?tcase_id=..&tproject_id=..', 'tcView_<id>', 900x700).

This mirrors legacy openExecHistoryWindow() / openTCaseWindow() (gui/javascript/testlink_library.js:1720/1003) using the established Dashio popup convention (tcAssignments.html:362-368, tcasesWithCF.html:102-110). i18n keys ta2p.execHistory + ta2p.design added to all 10 locale bundles.

• Both icons open the correct modern screens (execHistory.html, tcView.html) for the same test case/project; tooltips localize (tested with ?locale=ro).
• Add-flow regression passes; Event Viewer shows no new Error/Warning rows.
• Test suite recorded in tmp/TLU_Test_Cases.md (Task — Issue #1320, 6/6 PASS).

---

Legacy dashio/testcases/tcAssign2Tplan.tpl renders the Cancel button OUTSIDE the {if $gui->can_do} conditional (tpl line 86), so the user can always navigate away — even when every plan/platform row is read-only because the shown tcversion is already linked to a different version (can_do=false).

Modern gui/templates/testcases/tcAssign2Tplan.html kept both Add and Cancel inside the single #actionsBar div, and renderPlans() hid that div whenever ctx.info.can_do was false — trapping the user on a screen with no action row (only browser back / tab close), while the legacy UI always offered Cancel.

• tcAssign2Tplan.html:83-85 — Cancel moved into its own #cancelBar actions-bar div, separate from #actionsBar (Add).
• renderPlans() shows #cancelBar unconditionally whenever test plans exist and gates ONLY #actionsBar on ctx.info.can_do (:226-230).
• Fatal-error and no-plans paths hide #cancelBar (:180-185, :194-202) exactly like legacy (no plans → no_test_plans text, no form → no buttons).
• BFF api/tcassign2tplan/index.php buildGrid() unchanged — can_do already computed exactly as legacy (OR of drawable checkboxes).

• Browser can_do=false (A2P-2 v2, Plan A linked on v1): read-only checked disabled row, Add hidden, Cancel visible (#cancelBar flex, #actionsBar none).
• Browser can_do=true (A2P-1 unlinked): addable row, Add + Cancel both visible.
• Add-flow regression passes; after assigning, view reloads to can_do=false with Add hidden and Cancel still available.
• Event Viewer: no new Error/Warning entries (only INFO audit rows).
• Test suite recorded in tmp/TLU_Test_Cases.md (Task — Issue #1319, 6/6 PASS).