建立前端无障碍基线扫描与基础语义修复

[yeyixiang2007]

What to build

Add a repo-local accessibility baseline for the web frontend and fix a first batch of obvious semantic issues so accessibility work has executable guardrails instead of staying fully manual.

Acceptance criteria

• Axe-based frontend accessibility baseline tests exist for at least one shared interaction surface.
• Obvious missing accessible names or dialog semantics in key UI surfaces are fixed.
• The new baseline is linked to the broader A11y audit effort rather than pretending to replace it.

Relationship to ISSUE-120

This is a repo-grounded sub-slice that supports ISSUE-120 but does not replace a full site-wide accessibility review and remediation pass.

[yeyixiang2007]

Implemented in commit 167ee90. Added an axe-based frontend accessibility baseline test file, fixed accessible names on search controls and homepage banner controls, and added explicit dialog semantics to key UI modals. Verified with: pnpm --filter scustack-web test -- accessibility.test.ts and pnpm --filter scustack-web typecheck.