fix(a11y): resolve pre-existing color-contrast and link-name violations found by live smoke tests

[davidsneighbour]

Context

Discovered while implementing #1699 (live-site Playwright smoke tests). The new src/test/live/accessibility.spec.ts test runs axe-core (WCAG 2.1 AA) against /, /blog/, /connect/, and /find/ on production, and found real, pre-existing violations. The color-contrast rule is disabled in that test (and the one known link-name node is excluded) specifically to unblock #1699 without silently hiding these — this issue tracks fixing them for real.

Findings (production, kollitsch.dev)

color-contrast (serious) — all 4 pages:

• /: 22 nodes
• /blog/: 26 nodes
• /connect/: 9 nodes
• /find/: 11 nodes

Known contributors include the large decorative "KOLLITSCH.dev*" watermark heading (very low contrast by design — may be legitimately exempt as decorative/non-text, needs a design decision recorded in DESIGN.md either way) and several real link-contrast issues in the footer (RSS/Atom/JSON feed links, the Static.Quest web-ring links) at ~3.05:1 against a 4.5:1 requirement.

link-name (serious) — /blog/:
One node: the cover-image overlay link for the post /blog/2025/language-kuchisabishii/ renders aria-label="". Root cause: PostImage.astro sets aria-label={cover.alt}, and that post's frontmatter (src/content/blog/2025/language-kuchisabishii/index.md) has no cover.alt and an empty cover.title (content.config.ts's schema requires cover.title to be set before cover.alt is allowed, so a proper fix needs both fields populated with real photo details).

Suggested approach

1. Decide (and document in DESIGN.md) whether the watermark heading is intentionally exempt from contrast requirements as decorative/non-text content; if not, raise its contrast.
2. Fix footer link contrast (RSS/Atom/JSON/web-ring links) to meet 4.5:1 in both light and dark mode.
3. Add a real cover.title/cover.alt pair to language-kuchisabishii/index.md (needs the actual photo source/credit, which this triage didn't have).
4. Once fixed, remove the disableRules(['color-contrast']) call and the .exclude(...) for that post's link in src/test/live/accessibility.spec.ts so the live smoke test enforces the full WCAG 2.1 AA ruleset again.

Validation

npx playwright test --config ./playwright.live.config.ts --tsconfig ./tsconfig.playwright.json accessibility.spec.ts should pass with no rule exclusions.