fix: address code-review findings from pass 1 + 2 (#8)

- Fix saveCourseToDB returning 0 on conflict by falling back to SELECT
- Fix inactive layouts showing 'Never played' when last_played exists
- Add .icon-btn.spinning to courses.css for refresh button feedback
- Remove duplicate .btn-primary from courses.css (use shared.css version)
- Tokenize rating tier colors into --rating-tier-{high,mid,low} CSS vars
- Convert var to const/let throughout courses.js
- Fix logger.error calls to use {err} object form (pino convention)
- Extract RATING_TIER_HIGH/MID constants in course-layouts.ejs scriptlet
- Remove dead href='#' View all link from courses.ejs (deferred)
- Pass total prop explicitly from course-table.ejs to course-cards.ejs
- Remove dead #search-results-info selector from mobile.css
- Remove redundant .replace(/"/g, '"') from data attributes in course-table.ejs

public/css/shared.css

@@ -34,6 +34,11 @@
     --font-sans: 'Plus Jakarta Sans', system-ui, sans-serif;
     --font-mono: 'JetBrains Mono', ui-monospace, monospace;
 
+    /* ── Rating tier tokens ───────────────────────── */
+    --rating-tier-high:   oklch(0.55 0.15 150);
+    --rating-tier-mid:    oklch(0.55 0.12 100);
+    --rating-tier-low:    oklch(0.55 0.10 50);
+
     /* legacy token aliases — remove as components migrate */
     --surface-0: var(--bg);
     --surface-1: var(--paper);