fix(core,app,android,cli,packaging): harden account sign-in and watch-history sync

- Desktop shows Proton's CAPTCHA instead of failing the sign-in: a wry webview
  in a child process of the app (`pstr-app::verify`), confined to
  https://*.proton.me, hands the token back and the sign-in is retried. Linux
  packages and CI gain WebKitGTK 4.1; the Windows zip and MSI ship
  WebView2Loader.dll, without which a -gnu build does not start.
- The desktop keyring splits a secret too long for one entry across several,
  so a Proton session with many key salts survives Windows' 1280-unit cap.
  Short secrets stay one entry, so every existing secret still reads.
- `WatchSync` runs against a small `HistoryDrive` trait and is tested end to
  end against an in-memory Drive: two devices, newer wins, no re-upload of an
  unchanged history, a missing file written again, an unreadable own file
  never overwritten, a newer-format file left alone.
- An installation's file unwritten for 90 days is retired by the next device
  to sync, after its entries are merged and carried in that device's file.
- Removing a share keeps its watch state: sync would bring it back from the
  other devices anyway, and re-adding the share picks up where it left off.
- Watch history syncs once more on desktop exit (bounded to 3 s) and, on
  Android, through a WorkManager job enqueued when the app leaves the screen.
- The second-factor prompts say security keys are not supported.

android/app/src/main/java/io/narl/protonstream/ui/AccountUi.kt

@@ -194,13 +194,16 @@ internal fun SignInDialog(account: AccountUiState, actions: AccountActions, onDi
         text = {
             Column(verticalArrangement = Arrangement.spacedBy(8.dp)) {
                 when (state) {
-                    AccountState.SecondFactor -> OutlinedTextField(
-                        code,
-                        { code = it },
-                        label = { Text("Two-factor code") },
-                        singleLine = true,
-                        keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.NumberPassword),
-                    )
+                    AccountState.SecondFactor -> {
+                        OutlinedTextField(
+                            code,
+                            { code = it },
+                            label = { Text("Code from your authenticator app") },
+                            singleLine = true,
+                            keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.NumberPassword),
+                        )
+                        Text(SECURITY_KEY_NOTE, style = MaterialTheme.typography.bodySmall)
+                    }
                     AccountState.MailboxPassword -> {
                         Text(
                             "This account has a separate mailbox password. It unlocks your files.",
@@ -329,6 +332,11 @@ private fun VerificationDialog(url: String, onToken: (String) -> Unit, onDismiss
     }
 }
 
+/** Security keys need WebAuthn, which the sign-in here does not speak. */
+private const val SECURITY_KEY_NOTE =
+    "Security keys are not supported. If your account has only a security key, add an " +
+        "authenticator app in your Proton account settings to sign in here."
+
 /** Installed once per page, at start and again at finish in case the first ran too early. */
 private const val VERIFICATION_BRIDGE = """
 if (!window.__pstrVerification) {

android/app/src/main/java/io/narl/protonstream/ui/SharesScreen.kt

@@ -136,15 +136,15 @@ internal fun SharesScreen(
         )
     }
     removing?.let { share ->
-        // Asked, because it cannot be undone: the link's secret, its offline
-        // files and its watch positions all go (`Catalog::remove_share`).
+        // Asked, because it cannot be undone: the link's secret and its
+        // offline files go. Watch positions stay (`Catalog::remove_share`).
         AlertDialog(
             onDismissRequest = { removing = null },
             title = { Text("Remove ${share.name}?") },
             text = {
                 Text(
-                    "Its titles leave the library, and its offline episodes and watch " +
-                        "progress are deleted. Adding the link again starts it fresh.",
+                    "Its titles leave the library and its offline episodes are deleted. " +
+                        "Watch progress is kept, so adding the link again picks up where you left off.",
                 )
             },
             confirmButton = {