Add post image upload interface (#12)

Adds accessible drag-and-drop and browse controls, staged attachment editing, responsive lazy-loaded thread image grids, and UI regression coverage.

Reviewed-on: #12
Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/web/server_test.go

@@ -173,6 +173,11 @@ func TestRegisterLoginAsk(t *testing.T) {
 		`id="submit-progress"`,
 		`data-submit-once`,
 		`data-submit-button`,
+		`enctype="multipart/form-data"`,
+		`data-image-picker`,
+		`id="submit-images"`,
+		`accept="image/jpeg,image/png,image/webp"`,
+		`Add up to 4 JPEG, PNG, or WebP images.`,
 	} {
 		if !strings.Contains(rec.Body.String(), want) {
 			t.Fatalf("submit form missing %q: %s", want, rec.Body.String())