Add reading plans: catalog, progress tracking, and custom plans

Replaces the old single-book reading goal with a full Reading Plans page:
9 built-in plans (Whole Bible, OT/NT, topical) with cover photos and
color-themed cards, day-by-day checklists synced to the server per user,
inline "mark today read" in the Bible reader, and user-created plans with
their own book picker and an optional uploaded cover photo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

public/plan-covers/README.txt

@@ -0,0 +1,18 @@
+Drop a cover photo here for each built-in reading plan, named exactly:
+
+  bible-year.jpg      Whole Bible in a Year
+  bible-90.jpg        Whole Bible in 90 Days
+  nt-90.jpg           New Testament in 90 Days
+  nt-30.jpg           New Testament in 30 Days
+  gospels-30.jpg      The Gospels in 30 Days
+  psalms-30.jpg       Psalms in 30 Days
+  proverbs-31.jpg     Proverbs in 31 Days
+  pentateuch-60.jpg   The Pentateuch in 60 Days
+  wisdom-40.jpg       Wisdom Literature in 40 Days
+
+Landscape orientation, ~1200px wide, works best (cards crop to a 16:9 box).
+Any plan without a matching file just shows its colorful gradient + icon —
+nothing breaks if some are missing.
+
+See src/utils/readingPlans.js -> PIXABAY_SEARCH_LINKS for a Pixabay search
+link per plan to help pick one.