Testimonials: blend into page, inline nav arrows, drop dots

User wanted the testimonial section to feel "of the page" rather than
a card sitting on top of it, and to occupy less vertical space.

- Strip all chrome from .card--testimonial: transparent background,
  no border, no shadow, no hover lift. Same treatment applied in the
  swiss + cosmos overrides so per-theme backgrounds don't sneak back.
- Restructure the carousel markup to put the prev/next buttons
  directly beside the viewport (carousel--inline = flex row), no
  separate controls block, no dot indicators.
- Shrink the curly quote mark and the avatar (44 -> 32px); tighten
  internal gaps. Removed the rule above the attribution row.
- Keep healthy horizontal padding via clamp() so the arrows don't
  crowd the surrounding sections.
- Update aria-label on the carousel itself instead of the dots so
  screen readers still get position context.

assets/css/cosmos.css

@@ -572,7 +572,9 @@
 
 /* ───── testimonial / carousel ───── */
 :root[data-template="cosmos"] .card--testimonial {
-  background: linear-gradient(135deg, rgba(30, 22, 64, 0.55), rgba(14, 8, 38, 0.55));
+  background: transparent;
+  backdrop-filter: none;
+  -webkit-backdrop-filter: none;
 }
 :root[data-template="cosmos"] .testimonial__mark {
   color: var(--accent);