halkyon_app: instances, sign-in and sync plumbing for Halkyon's apps

What every Halkyon app's shell needs, taken from Halkyon Learn's and set
on halkyon_base's client: the instances a device keeps an account on,
the first start's tiles and where an instance keeps people's data, the
account switcher; signing in anonymously, with GitHub or an email code,
switching person and the start screen; the gates that build an app's
dependencies per instance and per person; and the sync service with its
status, Settings card and banner. Texts of its own in English and
German take the app's name; what only the app can say, it passes in.
Fakes for apps' tests are in testing.dart.

An instance's basics now name the app it serves (`app`, set by the
host's configuration), and an app accepts only an instance of its own.
SerialQueue and CoalescingRunner move into halkyon_core.

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/lib/l10n/halkyon_app_en.arb

@@ -0,0 +1,201 @@
+{
+  "@@locale": "en",
+  "tryAgain": "Try again",
+  "instanceWelcome": "Welcome to {app}",
+  "@instanceWelcome": {
+    "description": "Screen readers hear it for the wordmark on the first start.",
+    "placeholders": {
+      "app": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceBuiltInTagline": "Comes with the app",
+  "instanceAnother": "Another instance",
+  "instanceAnotherTagline": "Enter its address",
+  "instanceAddress": "Instance address",
+  "instanceAddressInvalid": "This isn't an address.",
+  "instanceAddressRequired": "Enter the address of an instance.",
+  "instanceContinue": "Continue",
+  "instanceUnreachable": "No instance of {app} answers at {name}.",
+  "@instanceUnreachable": {
+    "placeholders": {
+      "app": {
+        "type": "String"
+      },
+      "name": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceUseBuiltIn": "Use {name}",
+  "@instanceUseBuiltIn": {
+    "placeholders": {
+      "name": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceOpenFailed": "Couldn't open the app on {name}.",
+  "@instanceOpenFailed": {
+    "placeholders": {
+      "name": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceDataAndBackupsIn": "Data and backups: {place}",
+  "@instanceDataAndBackupsIn": {
+    "placeholders": {
+      "place": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceDataIn": "Data: {place}",
+  "@instanceDataIn": {
+    "placeholders": {
+      "place": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceBackupsIn": "Backups: {place}",
+  "@instanceBackupsIn": {
+    "placeholders": {
+      "place": {
+        "type": "String"
+      }
+    }
+  },
+  "instanceNoBackups": "No backups",
+  "instanceEuropeanUnion": "European Union",
+  "accountsTitle": "Accounts",
+  "accountAdd": "Add account",
+  "accountAddSubtitle": "Choose the instance for the account to add.",
+  "accountSignedIn": "Signed in",
+  "accountAnonymous": "Anonymous",
+  "accountRemoveTooltip": "Remove account",
+  "accountRemoveTitle": "Remove account?",
+  "accountRemoveMessage": "Removes the account on {name} from this device, with all the device holds of it. What it synced stays on {name}: sign in there again to have it. An anonymous account can't be signed into again.",
+  "@accountRemoveMessage": {
+    "placeholders": {
+      "name": {
+        "type": "String"
+      }
+    }
+  },
+  "accountRemoveConfirm": "Remove",
+  "sessionUnreachable": "Couldn't reach the server.",
+  "personDataFailed": "Couldn't open this device's data.",
+  "startSignedOut": "Signed out",
+  "startOn": "Start on {instance}:",
+  "@startOn": {
+    "placeholders": {
+      "instance": {
+        "type": "String"
+      }
+    }
+  },
+  "startWithGitHub": "Sign in with GitHub",
+  "startWithEmail": "Continue with email",
+  "startWithoutAccount": "Start without an account",
+  "startAnonymous": "Progress is saved on this device. Sign in to keep it.",
+  "startGitHubFailed": "Couldn't sign in with GitHub.",
+  "startNoGitHubAccount": "No account here signs in with this GitHub account yet. Start without an account, then add GitHub in Profile.",
+  "startOtherInstance": "Use another instance",
+  "emailSignInTitle": "Continue with email",
+  "emailSignInIntro": "We mail you a code to sign in. This instance keeps only a fingerprint of your address, never the address itself, and writes to it only when you ask for a code.",
+  "emailSignInAddress": "Email address",
+  "emailSignInSend": "Send code",
+  "emailSignInCodeSent": "We mailed a code to {address}. It's valid for 15 minutes.",
+  "@emailSignInCodeSent": {
+    "placeholders": {
+      "address": {
+        "type": "String"
+      }
+    }
+  },
+  "emailSignInResent": "A new code is on its way.",
+  "emailSignInCode": "Code",
+  "emailSignInContinue": "Continue",
+  "emailSignInResend": "Send a new code",
+  "emailSignInOtherAddress": "Use another address",
+  "emailSignInSwitchTitle": "Switch to the account of {address}?",
+  "@emailSignInSwitchTitle": {
+    "placeholders": {
+      "address": {
+        "type": "String"
+      }
+    }
+  },
+  "emailSignInSwitchConfirm": "Switch",
+  "emailSignInSwitchStay": "Stay anonymous",
+  "emailSignInTaken": "{address} already signs in to another account. To switch to it, sign out first.",
+  "@emailSignInTaken": {
+    "placeholders": {
+      "address": {
+        "type": "String"
+      }
+    }
+  },
+  "emailSignInUnavailable": "This instance doesn't offer signing in with email.",
+  "emailSignInNotAnAddress": "This isn't an email address.",
+  "emailSignInTooManyCodes": "Too many codes in the last hour. Try again later.",
+  "emailSignInWrongCode": "That code isn't right.",
+  "emailSignInTooManyTries": "Too many wrong codes. Ask for a new one.",
+  "emailSignInCodeExpired": "This code is no longer valid. Ask for a new one.",
+  "emailSignInOtherAddressLinked": "You already sign in with another address.",
+  "emailSignInMailFailed": "The instance couldn't send the mail. Try again later.",
+  "emailSignInFailed": "Couldn't reach the instance. Try again.",
+  "syncTitle": "Sync",
+  "syncNow": "Sync now",
+  "syncStateSyncing": "Syncing…",
+  "syncStateSynced": "Everything synced",
+  "syncStateFailing": "Sync is failing",
+  "syncStateNotYet": "Not synced yet",
+  "syncStateMismatch": "This app can't sync with {server}",
+  "@syncStateMismatch": {
+    "placeholders": {
+      "server": {
+        "type": "String"
+      }
+    }
+  },
+  "syncMismatchDetail": "Update the app. If it is up to date, the server needs an update. What you do meanwhile stays on this device and syncs afterwards.",
+  "syncLastCompleted": "Last completed at {time}",
+  "@syncLastCompleted": {
+    "placeholders": {
+      "time": {
+        "type": "String"
+      }
+    }
+  },
+  "syncLastFailed": "Last attempt failed at {time}",
+  "@syncLastFailed": {
+    "placeholders": {
+      "time": {
+        "type": "String"
+      }
+    }
+  },
+  "syncNoRunYet": "No sync has completed since the app started",
+  "syncPending": "{count, plural, =1{1 change waits to sync} other{{count} changes wait to sync}}",
+  "@syncPending": {
+    "placeholders": {
+      "count": {
+        "type": "int"
+      }
+    }
+  },
+  "syncTroubleLabel": "Sync needs attention",
+  "syncMismatchBanner": "This app can't sync with {server} any more. Update it to keep syncing.",
+  "@syncMismatchBanner": {
+    "placeholders": {
+      "server": {
+        "type": "String"
+      }
+    }
+  },
+  "syncMismatchMore": "More"
+}