fix(m5): modal layout for the test-template editor

The +New test modal capped at max-w-2xl rendered the 15-column MITRE matrix
in a 672px frame with no height cap, so the matrix spilled to the right of
the dialog, the form bottom dropped below the viewport, and neither scroll
direction worked — buttons were unreachable.

- Modal: add a `size` prop (default 2xl, back-compat) with a `7xl` preset.
  Cap height at calc(100vh-2rem), make the header sticky, and wrap children
  in a min-w-0 flex-1 overflow-y-auto body so tall content scrolls inside.
- MitreTagPicker: move overflow-x-auto from the grid itself to a dedicated
  scroller wrapper, and add `min-w-0` so the constraint propagates from the
  modal body. The grid's 1680px intrinsic min-width previously prevented
  the parent's overflow-x-auto from kicking in.
- AdminTestsPage: switch the form layout from `grid gap-3` to `flex flex-col
  gap-3 min-w-0` and set the modal size to 7xl. The CSS Grid form was
  propagating min-width: auto to all its items, which let the picker drag
  the body past the modal width.
- AdminScenariosPage: bump the modal to size 3xl for breathing room around
  the catalogue picker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -31,6 +31,12 @@ All notable changes to this project will be documented here. Format: [Keep a Cha
 - **`LogRecord` key collision**: `log.info(..., extra={"name": ...})` raises `KeyError("Attempt to overwrite 'name' in LogRecord")` because `name` is reserved by Python's stdlib logging. Renamed to `template_name`.
 - **React `currentTarget` null in deferred state updaters**: `onChange={(e) => setX((prev) => ({ ...prev, q: e.currentTarget.value }))}` blanked the page on the first user input because `currentTarget` is cleared after the listener bubble ends, before React invokes the updater. Switched all M5 handlers to `e.target.value`, which persists on the synthetic event.
 
+### Fixed (post-M5 UI — modal layout for the test-template editor)
+- **Modal box capped its width at `max-w-2xl` and had no vertical scroll** (`frontend/src/components/ui/Modal.tsx`): opening **+ New test** rendered the 15-column MITRE matrix inside a 672 px frame with no height cap, so the matrix spilled to the right and the form bottom dropped below the viewport — buttons unreachable, no scroll. Added a `size` prop (default `2xl` for back-compat), `max-h-[calc(100vh-2rem)]` + `flex flex-col` on the dialog, and an inner `min-w-0 flex-1 overflow-y-auto` body so the header stays pinned while the form scrolls inside the modal.
+- **MITRE matrix overflow-x failed to scroll inside the modal body** (`frontend/src/components/MitreTagPicker.tsx`): `overflow-x-auto` sat directly on the grid element, but the grid's intrinsic min-width (`15 × minmax(7rem, …)` = 1680 px) prevented it from shrinking below its content, so the grid spilled outside its parent instead of scrolling. Wrapped the grid in a dedicated `overflow-x-auto rounded min-w-0 w-full` scroller and added `min-w-0` to the picker root so the constraint propagates from the modal body. The grid now scrolls horizontally inside the modal.
+- **`grid gap-3` form layout in the test-template modal propagated `min-width: auto`** (`frontend/src/pages/AdminTestsPage.tsx`): each grid item refused to shrink below its widest child, so the picker dragged the form (and the body) past the modal width. Switched the form to `flex flex-col gap-3 min-w-0`, which breaks the propagation while preserving vertical spacing.
+- **Test-template modal now uses `size="7xl"`** and the scenario-template modal `size="3xl"` to match their content density.
+
 ### Fixed (post-M5 review pass — spec-reviewer + code-reviewer)
 - **Filter combinator was OR, not AND** (`backend/app/services/test_templates.py:235`): `?tactic=TA0002&technique=T1059` returned templates matching *either* facet instead of *both*. Pre-fix also pooled all three UUIDs into a shared `IN` list across three columns, theoretically allowing a UUID collision to match across kinds. Refactored to one IN-subquery per facet, ANDed together via repeated `WHERE id IN (...)`.
 - **Concurrent reorder race on `set_scenario_tests`** (`backend/app/services/scenario_templates.py:207`): two parallel reorders on the same scenario could deadlock on the `UNIQUE(scenario_id, position)` constraint under READ COMMITTED. Added a per-scenario `pg_advisory_xact_lock(0x5C3, hash(scenario_id))` mirroring the M4 `/mitre/sync` pattern; different scenarios don't contend.