Delete the legacy theme layer and dead code (#2660)

[jh-RLI]

What and why

The last ticket of the stage-2 design spec (#2632), and its contract step. Every
theme partial has now moved into scss/components/, so the legacy layer goes.
Dead code is deleted, not restyled.

Closes #2660

What changed

The legacy layer is gone. theming/scss/legacy/ held one last partial,
_controls.scss (.controls-container). It is deleted, along with its line in
oepstrap.scss, its REUSE entry and the text in theming/README.md and
docs/dev/frontend/design-system.md that described the layer. Only one of its
classes was still in a template: controls-container__tags on the table list's
tag select (dataedit/filter.html). It set flex: 1 inside #tags, which is
not a flex container, so it did nothing and is removed. The rebuilt
bootstrap.min.css loses only .controls-container* rules (892 bytes), checked
rule by rule.

Dead code deleted. I grepped each item across templates, Python, JS, SCSS
and every app's static/ dir before deleting it. That covered render,
template_name, {% include %} and Django's implicit class-based-view template
names.

Deleted	Why it was dead
login/: activate, detach, generic_form, oepuser_create_form	no view or include renders them
login/delete_account.html + AccountDeleteView_TODO_UNUSED (and its two now-unused imports)	the class was not routed. login:account-delete still answers 404, as intended; test_profile_owner_rule pins that
modelview/: overview, form_element_factory, widgets/dynamic_array, widgets/dynamic_edit, widgets/multiwidget	no view, include or widget names them
dataedit/: admin, url_list_field, tag_form	nothing renders them; no Create/UpdateView on Tag that would load tag_form implicitly
modelview/static/css/custom_alert.css	loaded by nothing
.panel-heading rules in mv-style.css (Bootstrap 3 glyphicons) and the rule-less fill-div class in modellist.html	no panel-heading anywhere. #2655 left both for this ticket
the user-select-none mixin	no @include
REUSE entries for all of the above, plus the one for legacy/_headers.scss	#2640 deleted that file but left its entry behind

The comment in base/tests/test_page_lint.py that listed the unrendered pages
"for #2660" is gone with them.

Found still in use, and kept:

• .reviewer-item (layouts/_review.scss). The issue lists it as dead, but
  it is not. updateFieldDescription in peer_review/ui/field_description.js
  builds it, and both peer review pages call that function
  (opr_reviewer.js:269, opr_contributor.js:210). This is the one item on the
  issue's list that is deliberately not deleted.
• .login__*. Every rule left in layouts/_login.scss (wrap, logo, heading,
  field, hint, separation, alternative, verification) has a user since Fields and account forms: one field look, allauth element overrides #2642.

Already gone before this branch: .btn--xs (#2641 inlined it), .pagination-nav
(#2647), the empty-state mixin (#2646), account/_login.html (#2642), and
tagged_field.html.

How it was tested

• Full suite, on its own test databases (test_oep_django_2660,
  test_oedb_2660): 2,284 tests, 106 skipped, 1 failure. The failure is
  oekg…LightImportTest, which fails under the isolated-settings setup because
  it overwrites PYTHONPATH for its subprocess. Run alone it passes. The page
  lint is part of the suite.
• check_catalogue: 20 entries, rendered in both token modes.
• Literal scan: 77 OEP colours, 0 painted under ?tokens=bootstrap.
• Theme: npm run build:theme on the base commit reproduces develop's
  files byte for byte. On the branch, stock_tokens.css is unchanged and
  bootstrap.min.css differs only by the removed rules.
• Homepage comparison (benchmarks/homepage/compare.mjs, 1440 and 390 px,
  signed out and in, whole page, plus the open menu and the burger). Before and
  after: 8 of 8 shots byte-identical, no difference. The server ran with
  COMPRESS_ENABLED = False and an empty STATIC_ROOT, and serves the rebuilt
  theme.
• reuse lint passes. mkdocs build passes (its one warning predates this
  branch).

Deploy notes

None.

For reviewers

• Start reading at: the commit message of 8b37d7fca. The diff is almost
  entirely deletions.
• Decisions I'd like checked: keeping .reviewer-item. The spec calls it
  dead; the code shows it is live.
• Deliberately left out:
  • dataedit/static/dataedit/metadata.js. No template or module loads
    it, so the whole file is dead. It is not on the issue's list, and a whole
    file deserves its own decision, so it is named here rather than deleted.
  • Bootstrap configuration that Underline tabs and house dropdowns #2648's review routed here:
    $dropdown-border-radius: 0 (overridden by components/_dropdown.scss),
    $nav-tabs-* (nav-tabs is retired and a source scan forbids it) and the
    .nav-pills remap in _remap.scss. These are not dead files. Changing
    them changes compiled values outside what the homepage comparison covers,
    and .nav-pills may still be live through the vendored jsoneditor on the
    metadata editor. Not on the issue's list either.
  • The account-delete route itself. It now only answers 404. Removing it
    or building the feature is a separate decision.

Checklist

Author

• 🐙 Followed the workflow in
  CONTRIBUTING.md
• 📝 Added a line to
  versions/changelogs/current.md
  under the right heading, ending with the PR number (#…)
• 📙 Updated the
  documentation, or none
  is needed
• 🧪 Added or updated tests, and the test suite passes (no new test: the
  change only deletes code)
• 🔌 Changed an API route or serializer? Regenerated the API reference (n/a)
• 🎨 UI change: uses components from the
  catalogue, or adds one with
  an entry (no visible change)
• 🤖 Used AI tools? Disclosed any changes you do not fully understand, as
  the
  AI covenant
  asks (written with Claude Code; every deletion re-checked by grep)
• 👀 Assigned a reviewer

Reviewer

• 🐙 Followed the
  reviewer guidelines
• 💻 Checked out the branch and ran it
• 💬 Gave feedback, and some appreciation for the work done

🤖 Generated with Claude Code