Redesign site and modernize tooling

Replace the HTML5-UP "Strata" theme with a Swiss-minimal single-page
design (plain CSS, Space Grotesk / Hanken Grotesk / Space Mono). Rewrite
copy for current positioning: Staff Data, Analytics & AI Engineer at
Swisscom, open to new work. Experience shown as a pipeline timeline;
contact simplified to email + LinkedIn (drop the broken form).

Prune unused deps (lightbox, FontAwesome, sass/gatsby-plugin-sass) and
the old SCSS, fonts and gallery assets. Fix package.json metadata.

Harden the Gitea deploy: merge build+deploy into one job (removes the
deprecated artifact upload/download handoff), bump Node 18 -> 20, use
npx gatsby build. Rename .nvrmc -> .nvmrc and refresh the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gatsby-config.js

@@ -1,24 +1,24 @@
 module.exports = {
   siteMetadata: {
-    title: "Dennis Thiessen - IT Software Engineering",
+    title: "Dennis Thiessen — Data, Analytics & AI Engineer",
     author: "Dennis Thiessen",
-    description: "Portfolio of Dennis Thiessen - IT Software Engineer Freelancer"
+    description:
+      "Dennis Thiessen is a Staff Data, Analytics & AI Engineer in Bern, Switzerland, building the data pipelines and ML infrastructure that production systems run on."
   },
   plugins: [
     'gatsby-plugin-react-helmet',
     {
       resolve: `gatsby-plugin-manifest`,
       options: {
-        name: 'gatsby-starter-default',
-        short_name: 'Dennis Thiessen',
+        name: 'Dennis Thiessen',
+        short_name: 'D. Thiessen',
         start_url: '/',
-        background_color: '#663399',
-        theme_color: '#663399',
+        background_color: '#f4f5f7',
+        theme_color: '#15171c',
         display: 'minimal-ui',
         icon: 'src/assets/images/website-icon-dt.png',
       },
     },
-    'gatsby-plugin-sass',
     'gatsby-plugin-offline'
   ],
 }