feat: data foundation (models, secure storage, Retrofit API, repository, L10n)

- java.time via core library desugaring (minSdk 24)
- Domain models: CalEvent, AppSettings, account/calendar models
- CredentialStore (EncryptedSharedPreferences) + SettingsStore
- Retrofit CalendarrApi covering all server endpoints
- CalendarRepository: auth, settings, accounts, events (org.json parsing
  for mixed-type payloads), writable calendars, visibility toggles
- Hilt DI (Moshi), dynamic-baseUrl ApiProvider, AuthInterceptor
- L10n (de/en) ported from iOS

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/main/java/com/scarriffle/calendarr/util/Dates.kt

@@ -0,0 +1,77 @@
+package com.scarriffle.calendarr.util
+
+import java.time.Instant
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.OffsetDateTime
+import java.time.ZoneId
+import java.time.ZoneOffset
+import java.time.format.DateTimeFormatter
+
+/**
+ * Date parsing/formatting that mirrors the iOS client (CalEvent.swift).
+ *
+ * The backend may produce any of these forms:
+ *   "2026-05-17"                    (all-day, date only)
+ *   "2026-05-17T10:00:00Z"
+ *   "2026-05-17T10:00:00+02:00"
+ *   "2026-05-17T10:00:00.000Z"
+ *   "2026-05-17T10:00:00"           (no timezone -> treat as UTC)
+ *   "2026-05-17 10:00:00+00:00"     (Python isoformat, space separator)
+ */
+object Dates {
+
+    private val zone: ZoneId get() = ZoneId.systemDefault()
+
+    private val spaceSep: DateTimeFormatter =
+        DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ssXXX")
+    private val noTz: DateTimeFormatter =
+        DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss")
+
+    /** Parse a server date string into an [Instant]. Returns null if unparseable. */
+    fun parse(raw: String, allDay: Boolean): Instant? {
+        val s = raw.trim()
+        if (s.isEmpty()) return null
+
+        if (allDay || (s.length == 10 && !s.contains('T'))) {
+            return runCatching {
+                LocalDate.parse(s.take(10)).atStartOfDay(zone).toInstant()
+            }.getOrNull()
+        }
+        // ISO with offset / Z
+        runCatching { return OffsetDateTime.parse(s).toInstant() }
+        runCatching { return Instant.parse(s) }
+        // Python isoformat with a space
+        runCatching { return OffsetDateTime.parse(s, spaceSep).toInstant() }
+        // No timezone -> UTC
+        runCatching {
+            return LocalDateTime.parse(s.take(19), noTz).toInstant(ZoneOffset.UTC)
+        }
+        // Last resort: just the date part
+        return runCatching {
+            LocalDate.parse(s.take(10)).atStartOfDay(zone).toInstant()
+        }.getOrNull()
+    }
+
+    /** Format an [Instant] for sending to the server. */
+    fun format(instant: Instant, allDay: Boolean): String {
+        return if (allDay) {
+            LocalDate.ofInstant(instant, zone).toString() // yyyy-MM-dd
+        } else {
+            // ISO8601 with Z, no fractional seconds (matches iOS isoBasic)
+            DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss'Z'")
+                .withZone(ZoneOffset.UTC)
+                .format(instant)
+        }
+    }
+
+    /** ISO8601 UTC string used for the events query range (`?start=&end=`). */
+    fun isoUtc(instant: Instant): String =
+        DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss'Z'")
+            .withZone(ZoneOffset.UTC)
+            .format(instant)
+
+    fun localDate(instant: Instant): LocalDate = LocalDate.ofInstant(instant, zone)
+
+    fun startOfDay(date: LocalDate): Instant = date.atStartOfDay(zone).toInstant()
+}