feat(landing): static site + nginx image + Gitea CI (landing + macOS DMG)

- landing/: the spaceshell.ru terminal-dark landing (index.html + screenshots),
  containerized as an nginx:alpine image (Dockerfile + nginx.conf with gzip and
  asset caching, VERSION, .dockerignore).
- .gitea/workflows/build.yaml: adapted from the coddykinder pipeline to this repo.
  Path-gated jobs — `landing` builds & pushes the nginx image to the Gitea
  registry on landing changes; `dmg` builds a universal (Intel + Apple Silicon)
  .dmg via `tauri build` on app/crates changes and uploads it as an artifact;
  Max notification summarizes both. Tags build everything (release).
- DOCS/landing/spaceshell-landing.md: build brief + copy + SEO meta.

Notes: the DMG job needs a self-hosted macOS runner labelled `macos` (Tauri
can't cross-compile macOS from Linux); the DMG is unsigned until Developer ID
secrets are wired. Landing image verified locally (HTTP 200, assets served).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

landing/nginx.conf

@@ -0,0 +1,27 @@
+server {
+    listen 80;
+    listen [::]:80;
+    server_name _;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    gzip on;
+    gzip_comp_level 6;
+    gzip_min_length 1024;
+    gzip_types text/plain text/css application/javascript application/json image/svg+xml;
+
+    # Long cache for fingerprint-free static assets; HTML stays revalidated.
+    location ~* \.(?:css|js|png|jpe?g|gif|svg|webp|woff2?)$ {
+        expires 7d;
+        add_header Cache-Control "public, immutable";
+        access_log off;
+    }
+
+    location = /index.html {
+        add_header Cache-Control "no-cache";
+    }
+
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}