Fix UI accessibility baseline defects

[OwenMcGirr]

Summary

• render repository symbols through the shared responsive grid primitive
• respect browser font-size preferences and use logical RTL-safe positioning/alignment
• raise border and disabled-control contrast to WCAG targets
• consolidate focus-visible styling into one shared 3px/2px treatment
• prevent mobile-dialog viewport overflow by replacing 100vw with 100%

This establishes the accessibility baseline for the later Tailwind redesign without changing routes or supported behavior.

Verification

• focused site tests: 49 passed
• full pnpm test: site 65 passed; server 86 passed and 1 skipped; database integration 1 passed; Rails 26 passed
• pnpm lint: passed with 3 pre-existing Fast Refresh warnings
• pnpm typecheck: passed
• pnpm build: passed
• docker compose config --quiet: passed
• git diff --check: passed
• axe: repository page and existing affected layout/component checks passed
• browser checks: 320px reflow, no horizontal overflow, exact dialog viewport fit, focus indicator, Escape restoration, scroll cleanup, and 800px navigation transition passed
• measured contrast: border/white 3.22:1, border/canvas 3.10:1, disabled text/background 6.53:1, focus/background above 10:1

Notes

PR #54 is unchanged. This PR intentionally preserves the existing visual design apart from the approved contrast corrections.

Closes #59