fix(ui): surface download failures, close platform gaps, clean up code layer

[everpcpc]

Stacked on #1199 (on #1198, on #1197) — review those first; this diff shrinks to batch-4 only once they merge.

Problem

Final batch from the UI review:

• Download failures were indistinguishable from in-progress states: the failure icon rendered in the same secondary gray everywhere, and series/read-list aggregates had no failed state at all, so a partially failed download looked like business as usual.
• Assorted reader/detail/settings rough edges: the DIVINA page-jump preview card didn't respond to taps (the PDF one did), alternate-title labels wrapped at a hardcoded 60pt column, the tap-zone preview read the deprecated UIScreen.main (wrong aspect in split view), EPUB theme settings used a lazy grid inside a self-sizing Form row (a known UIKit recursive-layout crash pattern), slider rows came in two layouts, the in-reader EPUB settings sheet showed full description text against the compact-sheet convention, the login form ignored Return on macOS, and several badges/overlays had adaptivity or clipping issues.
• tvOS gaps: network timeouts and retry counts were read-only, the cache page silently discarded invalid input, logs couldn't be searched or cleared, the Settings tab carried a redundant section level, the API-key copy button did nothing, and the delete-library confirmation (type-the-name alert) could never confirm because tvOS alerts can't host text input.
• macOS gaps: the two EPUB settings pages ignored the shared form width, the Settings window forced its sidebar back open half a second after the user hid it, and the Protected toggle was disabled without explanation when device authentication is unavailable.
• Code layer: ~8 inline Binding(get:set:) sites, dead code (an unreachable ServerView branch, unused properties), five copies of the same deep-link handler each racing a 0.1s delayed append, and hardcoded server-row gradient colors.

Fix

• DownloadStatus/SeriesDownloadStatus gained a displayColor (failed = red) and the aggregates a failed case — any failure with nothing in flight surfaces as red at book, series, and read-list level; the count delta path falls back to a full recount when a failure is involved. Download All on a failed aggregate retries the failures.
• Reader/detail polish: shared PageJumpPreviewCard with tap-to-select on DIVINA, baseline-grid alternate titles, container-geometry tap-zone preview, eager EPUB theme grid, unified slider rows, description text trimmed from the in-reader EPUB settings sheet, merged duplicated reader transitions and next/previous-book navigation.
• Settings/login/feedback: icon-preview app-icon picker, adaptive browse-preview placeholder, Return-key focus chain and submit on the login form (plus a 44pt scheme toggle), stable update-password button width, komf failures via the standard alert channel with a distinguishable image-error placeholder, formatted directory-browser errors, semantic title type sizes, a pulsing live-task dot, wrapping detail timestamps, adaptive 18+ age-rating badge colors, and a 2-line toast cap.
• tvOS: editable network page, invalid cache input flagged red, log search/clear, flattened Settings tab, hidden copy button, and a plain delete-library confirmation (type-to-confirm stays on iOS/macOS).
• macOS: .settingsFormWidth() on both EPUB pages, the sidebar toggle is user-controlled again (the detail column already expands), and a footer explains the disabled Protected toggle.
• Code layer: inline bindings lifted to computed properties (network side effects moved to onChange), dead code removed, one deepLinkRouting modifier shared by all five shells — reset and push now land in a single atomic NavigationPath assignment instead of a deferred append that could race the stack rebuild — and the server-green gradient moved into asset colors.

Validation

• make format
• make build — iOS, macOS, tvOS all pass
• make localize + misc/translate.py list — no missing keys (12 new keys translated for all 10 languages; 7 stale slider keys removed)
• repo-conventions skill updated (download aggregate failed rule, shared deep-link modifier)
• tvOS/macOS behavior changes are build-verified only; recommend a manual pass on device