chore: modernize Go 1.26, update deps, fix lint issues

- Update Go version from 1.19 to 1.26
- Update all dependencies to latest versions
- Fix staticcheck warnings (unused vars, types, functions)
- Remove unused svgToPng function
- Add GitHub Actions CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - run: go vet ./...
+      - run: go build ./...