Accessibility backlog: WCAG 2.1 AA follow-ups

[amykapernick]

Tracking issue for accessibility follow-ups from the WCAG 2.1 AA audit in #129 (a11y/audit-and-ci) that weren't fixed in that pass — either out of scope, needing design/content input, or on lower-priority (portal/admin) pages.

Fixes already made on that branch (landmarks, heading order, skip link, TalkOptionCard keyboard support, the voting page live region, jsx-a11y lint tightening, and cross-browser visual regression testing) are not repeated here — see #129's commit history.

Each item below is tracked as its own sub-issue with the relevant file(s), WCAG criterion, and suggested fix.

Roughly in priority order (public-facing first):

• A11y: /blog fails automated contrast checks (unstyled page) #131 — /blog fails automated contrast checks (unstyled page)
• A11y: blog post thumbnails can ship with no alt text (imageAlt not enforced) #132 — blog post thumbnails can ship with no alt text
• A11y: /blog and /blog/:slug lack visual/structural hierarchy #133 — /blog and /blog/:slug lack visual/structural hierarchy
• A11y: sponsor portal login error isn't announced to screen readers #134 — sponsor portal login error isn't announced to screen readers
• A11y: audit sponsor portal pages (portal.tsx, portal.profile.tsx, portal._index.tsx) #135 — audit sponsor portal pages
• A11y: agenda-planner drag-and-drop has no keyboard equivalent #136 — agenda-planner drag-and-drop has no keyboard equivalent
• A11y: target=_blank links don't indicate they open in a new tab #137 — target=_blank links don't indicate they open in a new tab
• A11y: spot-check third-party widget accessibility (Tito, SalesMate) #138 — spot-check third-party widget accessibility (Tito, SalesMate)
• A11y: manual color-contrast audit of theme tokens, including hover/focus states #139 — manual color-contrast audit of theme tokens, including hover/focus states
• A11y: extend e2e axe suite route coverage (blog posts, talk detail, portal, admin) #140 — extend e2e axe suite route coverage
• A11y: extend focus-visible e2e coverage beyond header + voting buttons #141 — extend focus-visible e2e coverage

[amykapernick]

Source PR for the fixes already made: #129 (branch a11y/audit-and-ci). Items already fixed there (landmarks, heading order, skip link, TalkOptionCard keyboard support, voting live region, jsx-a11y lint tightening) are not repeated in this backlog.