feat(backend): handle processing errors and add PWA routes

- Block file downloads and previews with a 424 StatusFailedDependency if file processing failed or the box has issues.
- Register routes for `/service-worker.js` and `/share-target` to support PWA features.
- Update README.md with an AI usage disclosure.

backend/libs/handlers/static_test.go

@@ -1,7 +1,11 @@
 package handlers
 
 import (
+	"encoding/json"
+	"net/http"
 	"net/http/httptest"
+	"os"
+	"path/filepath"
 	"testing"
 )
 
@@ -24,3 +28,76 @@ func TestSetStaticCacheHeaders(t *testing.T) {
 		}
 	}
 }
+
+func TestWebManifestIncludesShareTarget(t *testing.T) {
+	data, err := os.ReadFile(filepath.Join("..", "..", "static", "site.webmanifest"))
+	if err != nil {
+		t.Fatalf("ReadFile returned error: %v", err)
+	}
+	var manifest struct {
+		ShareTarget struct {
+			Action  string `json:"action"`
+			Method  string `json:"method"`
+			EncType string `json:"enctype"`
+			Params  struct {
+				Title string `json:"title"`
+				Text  string `json:"text"`
+				URL   string `json:"url"`
+				Files []struct {
+					Name   string   `json:"name"`
+					Accept []string `json:"accept"`
+				} `json:"files"`
+			} `json:"params"`
+		} `json:"share_target"`
+	}
+	if err := json.Unmarshal(data, &manifest); err != nil {
+		t.Fatalf("json.Unmarshal returned error: %v", err)
+	}
+	if manifest.ShareTarget.Action != "/share-target" || manifest.ShareTarget.Method != "POST" || manifest.ShareTarget.EncType != "multipart/form-data" {
+		t.Fatalf("unexpected share_target config: %+v", manifest.ShareTarget)
+	}
+	if manifest.ShareTarget.Params.Title != "title" || manifest.ShareTarget.Params.Text != "text" || manifest.ShareTarget.Params.URL != "url" {
+		t.Fatalf("unexpected share_target params: %+v", manifest.ShareTarget.Params)
+	}
+	if len(manifest.ShareTarget.Params.Files) != 1 || manifest.ShareTarget.Params.Files[0].Name != "files" || len(manifest.ShareTarget.Params.Files[0].Accept) != 1 || manifest.ShareTarget.Params.Files[0].Accept[0] != "*/*" {
+		t.Fatalf("unexpected share_target files: %+v", manifest.ShareTarget.Params.Files)
+	}
+}
+
+func TestServiceWorkerServedFromRootScope(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+
+	request := httptest.NewRequest(http.MethodGet, "/service-worker.js", nil)
+	response := httptest.NewRecorder()
+	app.ServiceWorker(response, request)
+
+	if response.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
+	}
+	if got := response.Header().Get("Service-Worker-Allowed"); got != "/" {
+		t.Fatalf("Service-Worker-Allowed = %q, want /", got)
+	}
+	if got := response.Header().Get("Content-Type"); got != "text/javascript; charset=utf-8" {
+		t.Fatalf("Content-Type = %q", got)
+	}
+	if response.Body.Len() == 0 {
+		t.Fatalf("service worker body missing")
+	}
+}
+
+func TestShareTargetFallbackRedirectsHome(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+
+	request := httptest.NewRequest(http.MethodPost, "/share-target", nil)
+	response := httptest.NewRecorder()
+	app.ShareTargetFallback(response, request)
+
+	if response.Code != http.StatusSeeOther {
+		t.Fatalf("status = %d, want %d", response.Code, http.StatusSeeOther)
+	}
+	if got := response.Header().Get("Location"); got != "/?share-target=unsupported" {
+		t.Fatalf("Location = %q", got)
+	}
+}