Fix release links in README: use absolute Gitea URLs, drop 'GitHub' wording

The relative '../../releases/latest' links did not resolve on Gitea. Point the
fabrication-data links at the absolute releases/latest URL and rename the
fabrication zip reference to esp-sensair-v1.0.0-gerbers.zip.

CHANGELOG.md

@@ -16,7 +16,7 @@ First public release of the ESP32-C3 CO₂ & environment sensor board (prototype
   opens with all custom parts resolved on a fresh clone.
 - **Fabrication data**: Gerber X2 plot, Excellon PTH/NPTH drills, Gerber
   job/stackup file, packaged as a ready-to-upload `esp-sensair.zip`. These are
-  derived artifacts published as GitHub Release assets (not committed to the tree).
+  derived artifacts published as release assets (not committed to the tree).
 - **Bill of materials** (`bom/esp-sensair-bom.csv`) plus the human-readable BOM
   in the README.
 - **Documentation** in `docs/`: schematic PDF, top/bottom layer plots, 3D render.