feat: blur unread book and series covers

[everpcpc]

Problem

Issue #618 asks for a way to hide spoiler-heavy covers until reading has actually started. The existing UI could show unread indicators, but unread book, oneshot, and series covers still rendered fully in browse and detail surfaces.

Approach

Add a browse appearance setting that blurs unread covers and apply it through the shared thumbnail renderer so only the image content is blurred while badges, overlays, and actions stay readable. Reading-state helpers are centralized on Book, KomgaBook, Series, and KomgaSeries so cover UI, detail actions, reader resume logic, and reading stats use the same unread/in-progress/completed semantics.

Scope

• Add thumbnailBlurUnreadCovers settings UI, preview support, and localized strings
• Blur unread covers for books, oneshots, and series in browse cards, rows, and detail headers
• Keep blur optional and avoid attaching blur modifiers when the radius is zero
• Consolidate repeated reading-state checks into shared computed properties

Validation

• make format
• make build

Closes #618.