fix: start a new account at zero unread

An absent view record read as unread, so on first sign-in every thread a list
had ever carried was unread. On a busy list that is thousands of threads the
user never intended to read, inflating the Home dashboard, the needs-attention
snapshot, and the widget with a number that means nothing.

Record a baseline when an account is activated: mail that predates it is read,
mail after it is not. activate() is the single funnel for launch validation,
account switching, and adding an account, so one call covers every sign-in path.

Accounts that already carry read state have been in use, so they get a
distantPast baseline and keep every unread thread they had — upgrading must not
silently mark a real backlog as read.

markUnread now records an explicit distantPast marker instead of deleting the
entry. Deleting would drop the thread back to the baseline rule, so marking an
old thread unread would appear to do nothing.

1 files changed, 104 insertions, 0 deletions

diff --git a/HutchTests/InboxViewModelTests.swift b/HutchTests/InboxViewModelTests.swift
index 7bfc199..92f380a 100644
--- a/HutchTests/InboxViewModelTests.swift
+++ b/HutchTests/InboxViewModelTests.swift
@@ -42,6 +42,110 @@ struct InboxViewModelTests {
     }
 
     @Test
+    func freshAccountTreatsExistingMailAsRead() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        let signIn = Date(timeIntervalSince1970: 5_000)
+        InboxReadStateStore.establishBaselineIfNeeded(now: signIn, defaults: defaults)
+
+        // Years of list history should not land on a new user as unread.
+        #expect(
+            !InboxReadStateStore.isUnread(
+                threadID: "list#old",
+                lastActivityAt: Date(timeIntervalSince1970: 4_000),
+                defaults: defaults
+            )
+        )
+    }
+
+    @Test
+    func mailArrivingAfterSignInIsUnread() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        InboxReadStateStore.establishBaselineIfNeeded(now: Date(timeIntervalSince1970: 5_000), defaults: defaults)
+
+        #expect(
+            InboxReadStateStore.isUnread(
+                threadID: "list#new",
+                lastActivityAt: Date(timeIntervalSince1970: 6_000),
+                defaults: defaults
+            )
+        )
+    }
+
+    @Test
+    func mailExactlyAtTheBaselineIsRead() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        let signIn = Date(timeIntervalSince1970: 5_000)
+        InboxReadStateStore.establishBaselineIfNeeded(now: signIn, defaults: defaults)
+
+        #expect(!InboxReadStateStore.isUnread(threadID: "list#edge", lastActivityAt: signIn, defaults: defaults))
+    }
+
+    @Test
+    func baselineIsEstablishedOnceAndNotMovedBySubsequentSignIns() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        InboxReadStateStore.establishBaselineIfNeeded(now: Date(timeIntervalSince1970: 5_000), defaults: defaults)
+        // A later launch must not silently mark the backlog read.
+        InboxReadStateStore.establishBaselineIfNeeded(now: Date(timeIntervalSince1970: 9_000), defaults: defaults)
+
+        #expect(
+            InboxReadStateStore.isUnread(
+                threadID: "list#since",
+                lastActivityAt: Date(timeIntervalSince1970: 6_000),
+                defaults: defaults
+            )
+        )
+    }
+
+    @Test
+    func existingAccountsKeepTheirUnreadBacklog() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        // An account already carrying read state has been in use, so upgrading
+        // must not retroactively mark everything it had not read as read.
+        InboxReadStateStore.markViewed(Date(timeIntervalSince1970: 1_000), for: "list#seen", defaults: defaults)
+        InboxReadStateStore.establishBaselineIfNeeded(now: Date(timeIntervalSince1970: 5_000), defaults: defaults)
+
+        #expect(
+            InboxReadStateStore.isUnread(
+                threadID: "list#unseen",
+                lastActivityAt: Date(timeIntervalSince1970: 4_000),
+                defaults: defaults
+            )
+        )
+    }
+
+    @Test
+    func markingAnOldThreadUnreadSurvivesTheBaseline() {
+        let suiteName = "InboxViewModelTests-\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        InboxReadStateStore.establishBaselineIfNeeded(now: Date(timeIntervalSince1970: 5_000), defaults: defaults)
+        let oldActivity = Date(timeIntervalSince1970: 4_000)
+
+        #expect(!InboxReadStateStore.isUnread(threadID: "list#old", lastActivityAt: oldActivity, defaults: defaults))
+
+        // Explicitly marking it unread must stick, rather than falling back to the
+        // baseline rule and reading as read again.
+        InboxReadStateStore.markUnread(for: "list#old", defaults: defaults)
+        #expect(InboxReadStateStore.isUnread(threadID: "list#old", lastActivityAt: oldActivity, defaults: defaults))
+    }
+
+    @Test
     func normalizesThreadSubjectsForDisplay() {
         let summary = InboxThreadSummary(
             rootEmailID: 1,