feat(tile-proxy): Thunderforest Outdoors as foreign karte upstream

OpenTopoMap's hypsometric tint reads "red mountains / green flats" and
looks nothing like the Swisstopo Pixelkarte the proxy hands out
in-region — produced a jarring visual seam right at the CH/LI border.
Thunderforest Outdoors has a muted topo palette + subtle hillshade
that matches the swisstopo tile aesthetic much more closely, so use
it as the abroad `karte` upstream when an API key is available.

- `tile-proxy/build.rs`: reads `tile-proxy/.env` (gitignored) at build
  time and forwards each `KEY=VAL` line to rustc as `--env`, so the
  key is baked into the binary via `option_env!` and never touched at
  runtime. A shell env var of the same name wins over the .env entry
  (dotenv precedence). `cargo:rerun-if-changed=.env` +
  `cargo:rerun-if-env-changed` force a recompile whenever the value
  changes — no stale key cached in the binary.
- `main.rs`: `THUNDERFOREST_API_KEY` read via `option_env!`; foreign
  `karte` is Thunderforest Outdoors when set, OpenTopoMap fallback
  when absent. Behaviour unchanged for keyless builds.
- `mapTiles.ts`: page-footer attribution credits Thunderforest + OSM
  alongside the existing swisstopo / OpenTopoMap / Esri lines so the
  attribution stays correct regardless of which build is deployed.
- `.gitignore`: tile-proxy build artefacts (binary, `target/`, `.env`)
  moved to the root gitignore with fully-qualified paths so the
  source tree isn't hidden by a nested gitignore quirk; the per-dir
  `tile-proxy/.gitignore` is removed.
- README + systemd service: documentation refreshed for the new
  build-time key flow.

src/lib/data/mapTiles.ts

@@ -21,10 +21,14 @@ export const TILE_URL = {
 } as const;
 
 /** Combined attribution — the proxy may serve any provider depending on the
- * region in view, so all three are credited. Shown in the page footer (the
- * on-map control is disabled). */
+ * region in view, so all are credited. Shown in the page footer (the on-map
+ * control is disabled). Thunderforest is the primary `karte` upstream abroad
+ * when the proxy was built with a `THUNDERFOREST_API_KEY`; OpenTopoMap is
+ * the no-key fallback. Both are credited so the attribution stays correct
+ * regardless of which build is deployed. */
 export const TILE_ATTRIBUTION =
 	'&copy; <a href="https://www.swisstopo.admin.ch/" target="_blank" rel="noopener">swisstopo</a> · ' +
-	'&copy; <a href="https://www.openstreetmap.org/copyright" target="_blank" rel="noopener">OpenStreetMap</a>, ' +
+	'Maps &copy; <a href="https://www.thunderforest.com/" target="_blank" rel="noopener">Thunderforest</a>, ' +
+	'Data &copy; <a href="https://www.openstreetmap.org/copyright" target="_blank" rel="noopener">OpenStreetMap</a> · ' +
 	'<a href="https://opentopomap.org/" target="_blank" rel="noopener">OpenTopoMap</a> · ' +
 	'&copy; <a href="https://www.esri.com/" target="_blank" rel="noopener">Esri</a>';