Own all form styling in components; remove form CSS from input.css

Form controls were styled "at a distance": Django renders bare
<input>/<select>/<textarea>/<label>, so input.css reached in with ID-scoped
#add-form descendant rules plus a global form *:disabled rule and .errorlist.
The #add-form ID specificity forced state rules to climb, needed
:not([data-search-select-search]) carve-outs, and broke on markup changes — it
surfaced as the add_purchase Name/related_game fields not reading as disabled.

Components now own all form styling via utilities on the elements themselves:

- PrimitiveWidgetsMixin stamps INPUT/SELECT/TEXTAREA_CLASS (incl. disabled:
  variants) onto native widgets by type, skipping SearchSelect (self-styled)
  and checkboxes.
- New FormFields(form, *, extras=...) renders label + control + errors + row
  layout with their own classes (replaces form.as_div()); the <form> owns its
  flex layout. extras appends a node into a named field's row (session
  timestamp buttons).
- AddForm/purchase/session render via FormFields; login too — a new
  LoginForm(PrimitiveWidgetsMixin, AuthenticationForm) styles its inputs and
  auth.py renders it via FormFields + a StyledButton (was as_table).
- input.css loses the entire #add-form block, the global :disabled rule, and
  .errorlist. State (disabled:) now lives on the element — no specificity wars,
  no carve-outs, robust to markup edits.

Tests: error rendering uses the component class (not .errorlist); add-form
labels/inputs carry their own classes; e2e login fixtures click the Login
button by text (submit is now a <button>); Name disabled cursor asserted.
CLAUDE.md documents the no-styling-at-a-distance + FormFields conventions.

513 passed; lint/format/ts-check clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

e2e/test_purchase_e2e.py

@@ -106,7 +106,7 @@ def authenticated_page(live_server, page: Page, django_user_model) -> Page:
     page.goto(f"{live_server.url}{reverse('login')}")
     page.fill('input[name="username"]', "tester")
     page.fill('input[name="password"]', "secret123")
-    page.click('input[type="submit"]')
+    page.click('button:has-text("Login")')
     page.wait_for_url(f"{live_server.url}/tracker**")
     return page