perf(mailbox): bound paging window (maxSize) and memoize per-row timestamp

Two fixes from the Paging 3 perf audit of the mailbox list (#212, #213):

- Bound the unified-inbox paging window (#212): the only PagingConfig left
  maxSize at Int.MAX_VALUE, so pages were never evicted and a deep scroll
  accumulated the whole inbox in memory. Set maxSize = MAILBOX_PAGE_SIZE * 5
  (200), satisfying maxSize >= pageSize + 2*prefetchDistance (120). Safe with
  enablePlaceholders = false (the UI null-guards evicted positions).

- Memoize the per-row relative timestamp (#213): MessageRow formatted it via
  DateUtils.getRelativeTimeSpanString un-remembered, allocating a String on
  every recomposition. Wrapped in remember(timestampMillis).

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

app/src/main/kotlin/org/libremail/data/repository/MailRepositoryImpl.kt

@@ -97,6 +97,11 @@ class MailRepositoryImpl @Inject constructor(
             pageSize = MAILBOX_PAGE_SIZE,
             initialLoadSize = MAILBOX_PAGE_SIZE * 3,
             enablePlaceholders = false,
+            // Bound the in-memory window so a deep scroll can't accumulate the whole (potentially
+            // thousands-of-rows) inbox: keep ~5 pages resident and evict the rest. Must be
+            // >= pageSize + 2*prefetchDistance (40 + 2*40 = 120); with placeholders off, evicted leading
+            // positions drop from the loaded window and the list rendering already null-guards them.
+            maxSize = MAILBOX_PAGE_SIZE * 5,
         ),
         pagingSourceFactory = { messageDao.pagingUnifiedFolderSummaries(folder) },
     ).flow.map { page -> page.map { it.toDomain() } }

app/src/main/kotlin/org/libremail/ui/mailbox/MailboxScreen.kt

@@ -549,7 +549,9 @@ private fun MessageRow(
                     Spacer(Modifier.width(4.dp))
                 }
                 Text(
-                    text = formatTimestamp(message.timestampMillis),
+                    // remember so the relative-time string isn't re-formatted (allocating a new String
+                    // and reading System.currentTimeMillis()) on every recomposition of the row.
+                    text = remember(message.timestampMillis) { formatTimestamp(message.timestampMillis) },
                     style = MaterialTheme.typography.labelSmall,
                     color = MaterialTheme.colorScheme.onSurfaceVariant,
                 )