feat(core,app,android,ios): look at shares again on their own, and say when they were last looked at

No client crawled a share unless someone pressed Refresh, so a new
episode stayed out of the library until then, and nothing said how stale
the library was.

Catalog schema v11 adds share_crawls: when each share's last crawl
finished and was attempted, how many files it stored and how many were
new, and why the last attempt failed. A failure keeps the last good
crawl's numbers.

Every client now crawls, quietly, any share not looked at for six hours:
the desktop at launch and every half hour while open, the phones at
launch and on their existing catch-up timer. A quiet crawl says only
news ("anime: 3 new files") and leaves failures on the share's row
rather than in a toast.

Share rows on all three show "120 files - crawled 2 h ago", or the last
failure until a crawl succeeds.

android/app/src/test/java/io/narl/protonstream/ui/ScreenshotTest.kt

@@ -309,9 +309,17 @@ class ScreenshotTest {
                 },
                 shelves = listOf(ShelfRecord("From MAPPA", listOf("oshi-no-ko", "bleach", "akira"))),
             ),
+            // Crawled relative to the clock, so the row reads "2 h ago" on
+            // whatever day the test runs.
             shares = listOf(
-                ShareRecord("anime", "anime", true),
-                ShareRecord("films", "films", false),
+                ShareRecord(
+                    "anime",
+                    "anime",
+                    true,
+                    crawledAt = System.currentTimeMillis() / 1_000 - 2 * 3_600,
+                    crawlFiles = 120uL,
+                ),
+                ShareRecord("films", "films", false, crawlError = "the link was revoked or has expired"),
             ),
             storage = StorageUsageRecord(6_400_000_000uL, 3uL, 0uL, 1_100_000_000uL),
         )