fix(ui): recount failed-download aggregates and restore card icon colors

[everpcpc]

Problem

Follow-up to the four-batch UI review (#1197–#1200), fixing two regressions that round introduced, plus the nits called out in the post-merge review:

• Stuck red failure state after Cancel/Retry All Failed. retryFailedBooks/cancelFailedBooks rewrite book-level downloadStatusRaw in bulk without recounting the series/read-list rollups. With the new aggregate failed state, Cancel All Failed left the aggregate pinned at failed (red icon stuck until some unrelated download event happened to trigger a recount), and Retry All Failed violated the "any pending → pending" rule until each book's next status change.
• Gray download icons on tinted/overlay cards. DownloadStatus.displayColor is non-optional (.secondary for everything but failures), so the downloadColor ?? palette.metaColor / ?? style.secondaryColor fallbacks in HorizontalCardSkeleton and GridCardView never fired — horizontal cards on a cover tint lost their white-70% accessory color, and overlay-mode grid cards lost their white-85% icon color.
• Dashboard section detail pages for local sections (Keep Reading, On Deck) told users to "Try selecting a different library." when empty, which makes no sense for server-independent sections.
• The macOS EPUB theme page still used the old combined-label sliders (the iOS page was unified in the earlier batch), which also forced the old Label: %@x keys to stay.
• The DIVINA page-transition picker's caption swap lost its animation when the broad Form-level animations were scoped down.
• macOS scroll arrows advanced only about half a page from the second click on (index computed from the leading edge while scrolling anchored to the center).
• Heatmap hit containers (28pt on a 17pt pitch) overlapped by 11pt, so each tile's trailing edge selected its neighbor.
• A failed book's download action read "Make Offline", indistinguishable from a never-downloaded book.

Fix

• After the bulk status writes, recount the rollups for every affected series and every read list containing the books, inside the same transaction (shared recountDownloadAggregates helper; bulk writes bypass the per-book delta path).
• Add failureColor: Color? (red only on failure, nil otherwise) to DownloadStatus/SeriesDownloadStatus and pass it at the five palette-surface call sites, so tinted/overlay cards keep their palette colors and only failures turn red. Concrete surfaces (chips, rows, status lines) keep the non-optional displayColor.
• Local dashboard sections now show "Nothing here yet." when empty (new key, translated for all 10 languages).
• Both EPUB theme pages share one LabeledSliderRow component (label + trailing value above, slider below); the seven stale Label: %@x keys are removed.
• The page-transition picker animates its caption swap via an explicit withAnimation binding (the border-cropping picker needs none — its caption is static).
• Scroll-arrow index is computed from the viewport center with truncation, so every click advances a full page.
• Heatmap hit containers now span exactly one grid pitch (17pt) with zero overlap — gaps stay tappable and no tap can select the wrong day.
• A failed download's action reads "Retry Download" (new key, translated for all 10 languages), keeping the button's action-color semantics intact.

Validation

• make format
• make build — iOS, macOS, tvOS all pass
• make localize + misc/translate.py list — no missing translations