feat(todo,app): files attached to tasks and comments

Halkyon's ADR 0043, D1, D3, D5 and D8: any file up to 100 MB, attached to
a task's description or to a comment, sealed as everything of Todo's.

Server: the attachment table, synced and sealed as the others, with the
sealed file's hash readable for the store; todoFiles hands halkyon_base
its rows, readable by their space's members, and takes sealed files
alone; the files no attachment names go daily. Its tests hold downloads
to the row's space, refuse a readable file and prune what no live row
names. Migration attachments: CREATE TABLE "attachment" with its indexes
and the foreign key to its space, cascading as every synced table's.

App: AttachmentRepository seals each file for its row, keeps it sealed
and opened in the person's folder, uploads it after each sync and fetches
it as it shows, both held back by Without files on mobile data but for a
file tapped. Under the description and a comment's text, a row of them:
pictures as thumbnails opening full-size, other files as chips opening in
the system's app, else saved; the paperclip beside the description and in the comment's
composer adds them, the composer holding them until it is sent. A task deleted takes its files
and its comments' along, a comment its own.

halkyon_app opens a file in the system's app (FileOpener): on Android
through its plugin and a FileProvider of its own class, sharing the
cache's halkyon_opened/ folder alone; on the desktop as a file: URI.

Whats-New: Attach files to a task's description or a comment with the paperclip: pictures show as thumbnails, other files open in their app.
Whats-New-De: Hänge mit der Büroklammer Dateien an die Beschreibung einer Aufgabe oder an einen Kommentar: Bilder erscheinen als Vorschau, andere Dateien öffnen sich in ihrer App.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/android/build.gradle.kts

@@ -20,3 +20,8 @@ android {
         targetCompatibility = JavaVersion.VERSION_17
     }
 }
+
+dependencies {
+    // FileProvider, for a file opened in another app (ADR 0043).
+    implementation("androidx.core:core:1.13.1")
+}

packages/halkyon_app/android/src/main/AndroidManifest.xml

@@ -1,4 +1,18 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
     <!-- Which network the device is on, and whether it is metered (ADR 0042). -->
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
+    <application>
+        <!-- A file opened in the app the system picks (`SystemFileOpener`,
+             ADR 0043): handed over as a content URI, readable by that app
+             alone, for that file alone. -->
+        <provider
+            android:name="app.halkyon.halkyon_app.HalkyonFileProvider"
+            android:authorities="${applicationId}.halkyon_app.files"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/halkyon_app_opened_files"/>
+        </provider>
+    </application>
 </manifest>

packages/halkyon_app/android/src/main/java/app/halkyon/halkyon_app/HalkyonAppPlugin.java

@@ -1,6 +1,8 @@
 package app.halkyon.halkyon_app;
 
+import android.content.ActivityNotFoundException;
 import android.content.Context;
+import android.content.Intent;
 import android.net.ConnectivityManager;
 import android.net.Network;
 import android.net.NetworkCapabilities;
@@ -8,10 +10,12 @@ import android.os.Build;
 import android.os.Handler;
 import android.os.Looper;
 import androidx.annotation.NonNull;
+import androidx.core.content.FileProvider;
 import io.flutter.embedding.engine.plugins.FlutterPlugin;
 import io.flutter.plugin.common.EventChannel;
 import io.flutter.plugin.common.MethodCall;
 import io.flutter.plugin.common.MethodChannel;
+import java.io.File;
 import java.util.HashMap;
 import java.util.Map;
 
@@ -25,6 +29,8 @@ import java.util.Map;
 public final class HalkyonAppPlugin
     implements FlutterPlugin, MethodChannel.MethodCallHandler, EventChannel.StreamHandler {
   private MethodChannel methods;
+  private MethodChannel files;
+  private Context context;
   private EventChannel events;
   private ConnectivityManager connectivity;
   private ConnectivityManager.NetworkCallback callback;
@@ -32,18 +38,20 @@ public final class HalkyonAppPlugin
 
   @Override
   public void onAttachedToEngine(@NonNull FlutterPluginBinding binding) {
-    connectivity =
-        (ConnectivityManager)
-            binding.getApplicationContext().getSystemService(Context.CONNECTIVITY_SERVICE);
+    context = binding.getApplicationContext();
+    connectivity = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
     methods = new MethodChannel(binding.getBinaryMessenger(), "app.halkyon/network");
     methods.setMethodCallHandler(this);
     events = new EventChannel(binding.getBinaryMessenger(), "app.halkyon/network/changes");
     events.setStreamHandler(this);
+    files = new MethodChannel(binding.getBinaryMessenger(), "app.halkyon/files");
+    files.setMethodCallHandler(this::onFileCall);
   }
 
   @Override
   public void onDetachedFromEngine(@NonNull FlutterPluginBinding binding) {
     methods.setMethodCallHandler(null);
+    files.setMethodCallHandler(null);
     events.setStreamHandler(null);
     onCancel(null);
   }
@@ -58,6 +66,34 @@ public final class HalkyonAppPlugin
     }
   }
 
+  /**
+   * Opens the file at `path` (in the cache's `halkyon_opened/`, the provider's
+   * one folder) as `mimeType` in the app the system picks; whether one took
+   * it.
+   */
+  private void onFileCall(@NonNull MethodCall call, @NonNull MethodChannel.Result result) {
+    if (!"open".equals(call.method)) {
+      result.notImplemented();
+      return;
+    }
+    String path = call.argument("path");
+    String mimeType = call.argument("mimeType");
+    try {
+      Intent intent =
+          new Intent(Intent.ACTION_VIEW)
+              .setDataAndType(
+                  FileProvider.getUriForFile(
+                      context, context.getPackageName() + ".halkyon_app.files", new File(path)),
+                  mimeType)
+              .addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_ACTIVITY_NEW_TASK);
+      context.startActivity(intent);
+      result.success(true);
+    } catch (ActivityNotFoundException | IllegalArgumentException none) {
+      // No app opens it, or it is outside the provider's folder.
+      result.success(false);
+    }
+  }
+
   @Override
   public void onListen(Object arguments, EventChannel.EventSink sink) {
     onCancel(null);

packages/halkyon_app/android/src/main/java/app/halkyon/halkyon_app/HalkyonFileProvider.java

@@ -0,0 +1,10 @@
+package app.halkyon.halkyon_app;
+
+import androidx.core.content.FileProvider;
+
+/**
+ * halkyon_app's own FileProvider (ADR 0043): a class of its own, since a
+ * manifest may name a provider class once, and an app (Train's GPX) or another
+ * plugin may declare androidx's own.
+ */
+public final class HalkyonFileProvider extends FileProvider {}

packages/halkyon_app/android/src/main/res/xml/halkyon_app_opened_files.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- What halkyon_app's FileProvider hands to another app: the copies made to
+     be opened (`SystemFileOpener`), in the cache's own folder, nothing else. -->
+<paths>
+    <cache-path name="opened" path="halkyon_opened/"/>
+</paths>