fix(ui): unify reader overlays, empty/error states, and hit targets

[everpcpc]

Stacked on #1197 — review that one first; this diff shrinks to batch-2 only once it merges.

Problem

Second batch from the UI review: the PDF reader overlay never received the chrome polish the DIVINA reader got, empty/error states had grown into four or five hand-drawn variants, and a sweep of plain buttons was only tappable on their glyphs (AGENTS.md rule 23).

• PDF chrome visibility animated with a single .easeInOut + insertion transition while DIVINA rides a split curve/spring; PDF page numbers hard-cut instead of .numericText(); PDF scrims lacked location stops and the 120pt readability extension past the bars.
• The DIVINA page-jump button stayed visibly tappable with no pages and silently did nothing.
• EPUB in-page progress filled from the leading edge even for RTL publications, and its download detail hand-formatted megabytes.
• Reader-level failures were drawn four different ways (NoPagesView with hardcoded white text, hand-rolled PDF and EPUB error rows, a one-line EPUB empty state), and app empty states used three different icon treatments; AdminRequiredView and the oneshot error state were hand-drawn too.
• ~20 plain buttons/links/menus (search results, TOC disclosures, preset apply, offline task actions, protection chips, membership rows, settings links, saved-filter apply, edit-sheet trash buttons, heatmap cells, section-header menus) had hit areas far below 44pt, and several icon-only buttons read their SF Symbol names to VoiceOver.

Fix

• Port the DIVINA chrome structure to the PDF overlay: opacity on .appCurve(0.3), bar slide on .appSpring, always-rendered bars driven by offset instead of insertion transitions, scrim stops with a 120pt extension, .numericText() page transitions, and .accessibilityHidden on hidden chrome.
• Disable the DIVINA page-jump button when the book has no pages.
• Mirror the EPUB in-page progress track for RTL publications and switch the EPUB download detail to ByteCountFormatter.
• Converge all book-level reader failures on ReaderUnavailableView (now with an optional prominent Retry; Close demotes to bordered when Retry is present), deleting NoPagesView.
• Render empty/error states through ContentUnavailableView everywhere: BrowseStateView internally (API unchanged), library list, server reading stats, admin-required, oneshot detail (with Retry), collection series query, picker-sheet no-results via ContentUnavailableView.search, and the PDF search sheet's three states.
• Add .contentShape (inside labels, per rule 23), 44pt minimum frames, and accessibility labels across the swept buttons; heatmap cells keep their 13pt visuals but gain 28pt hit containers with compensated grid pitch.
• Document the reader-level unavailable state and the empty/error state conventions in the repo-conventions skill.

Validation

• make format
• make build — iOS, macOS, tvOS all pass
• make localize + misc/translate.py list — no missing keys (2 new keys translated for all 10 languages)