feat: a newer app is told in a snackbar on Home and installed from a sheet holding Settings' card, fetched ahead on Wi-Fi (#6452)

Mirrors Librarian (#6446, #6448), after Gallery's update tray (canon #6367).

- Home shows "Version X of the app is on your server." / "…is downloaded
  and ready." with Update / Install; the button opens a bottom sheet
  holding the same About card Settings shows, so there is one install
  path. Each build is told at most once a day, kept on disk.
- The shell-wide update banner and its in-memory dismissals are gone.
- AppUpdates (singleton) holds the offer and the install, so Home and
  Settings show the same state. Checks run when either screen opens, on
  every return to the front, every 5 minutes while shown, and on Home's
  pull to refresh.
- UpdateFetchWorker downloads the newer APK ahead on an unmetered network
  (battery and storage not low, three tries); the download writes a .part
  file and keeps it under the build's name only once its size and sha256
  match, so Install is usually one tap.
- Back from allowing installs, the update the listener asked for carries
  on (kept in SavedStateHandle) instead of asking them to tap again.
- An offer needs a higher code AND a different name: the same app built
  again at a later minute is not offered.
- Server: /api/client/version reports the APK's sha256, worked out from
  the served file and cached by size and mtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/client_assets_test.go

@@ -2,6 +2,8 @@ package api
 
 import (
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"io"
 	"log/slog"
@@ -128,6 +130,39 @@ func TestClientVersion_200WithBothFiles(t *testing.T) {
 	if resp.SizeBytes != size {
 		t.Errorf("size_bytes: want %d, got %d", size, resp.SizeBytes)
 	}
+	want := sha256.Sum256([]byte("fake apk content"))
+	if resp.SHA256 != hex.EncodeToString(want[:]) {
+		t.Errorf("sha256: want the served file's digest %x, got %q", want, resp.SHA256)
+	}
+}
+
+// The digest is cached by size and modification time; a new APK dropped in
+// place must be answered with its own digest, not the old one's.
+func TestClientVersion_DigestFollowsAReplacedAPK(t *testing.T) {
+	writeClientAssets(t, `{"name":"a","code":1}`)
+	first := decodeClientVersion(t, getClientVersion(t))
+	dir := os.Getenv("MINSTREL_CLIENT_APK_DIR")
+	apk := filepath.Join(dir, clientAPKFilename)
+	if err := os.WriteFile(apk, []byte("another, longer build"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	second := decodeClientVersion(t, getClientVersion(t))
+	want := sha256.Sum256([]byte("another, longer build"))
+	if second.SHA256 != hex.EncodeToString(want[:]) || second.SHA256 == first.SHA256 {
+		t.Errorf("sha256 after replace: want %x, got %q (before: %q)", want, second.SHA256, first.SHA256)
+	}
+}
+
+func decodeClientVersion(t *testing.T, rr *httptest.ResponseRecorder) clientVersionResponse {
+	t.Helper()
+	if rr.Code != http.StatusOK {
+		t.Fatalf("want 200, got %d (body: %s)", rr.Code, rr.Body.String())
+	}
+	var resp clientVersionResponse
+	if err := json.Unmarshal(rr.Body.Bytes(), &resp); err != nil {
+		t.Fatal(err)
+	}
+	return resp
 }
 
 // A release published before ordering keys were recorded has a name and