Library: "Continue where you left off" row with a per-user setting

One fixed row above the library showing the five most recently read,
unfinished manga with the normal tiles (they stay in the grid as well).
It ignores search and filters, hides while sorted by "Recently read",
and can be turned off under Settings > Reading > Library page
(continueRow, default on).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -128,6 +128,7 @@ This keeps ids human-readable and derivable without a database. The price: renam
         "flowPageGap": true,            // show the 8 px separator between pages in Flow
         "readingClock": false,           // opt in to credible per-chapter active-time tracking
         "readingStory": true,            // show private retrospective summaries when history exists
+        "continueRow": true,             // library: "Continue where you left off" row at the top
         "zoom": { "flow": 1, "page": 1.5, "book": 1, "strip": 0.8 },   // page zoom per mode
         "webhook": ""
       }