Improve semantic navigation and icon-button accessibility

[MoerAI]

Summary

Several navigation and icon-only controls violate basic semantic/accessibility expectations: clickable cards/spans are used for navigation, and some icon-only buttons lack accessible labels.

Evidence

Clickable non-button/non-link navigation:

• src/app/(main)/admin/page.tsx:36 uses clickable Card for task navigation.
• src/app/(main)/reviewer/page.tsx:15 uses clickable Card for task navigation.
• src/app/(main)/worker/page.tsx:13 uses clickable Card for task navigation.
• src/components/common/RoleTabNav.tsx:40-43 uses a clickable span for home navigation.

Icon-only buttons missing aria-label:

• src/components/common/RoleTabNav.tsx:74-76 sign-out button.
• src/app/(main)/admin/tasks/create/page.tsx:367-373 remove option button.
• src/app/(main)/admin/groups/[groupId]/page.tsx:197-203 remove member button.

Impact

Keyboard users and screen-reader users may not be able to discover or operate these controls reliably. Navigation via clickable cards/spans also loses standard link behavior such as keyboard activation, focus semantics, and open-in-new-tab support where applicable.

Minimal Fix

• Use <Link> for navigation where possible.
• Use <button> for actions and ensure keyboard/focus states are visible.
• Add aria-label to icon-only buttons.
• Preserve existing visual styling while changing semantics.

Acceptance Criteria

• Task cards and app title navigation are reachable and operable by keyboard.
• Icon-only controls have meaningful accessible names.
• Visual appearance remains materially unchanged.
• npm run lint remains clean after changes.