Add bookmark/like support with floating heart button

Created by: jedmund

Summary

• Add POST endpoint support to PixivEndpoint (bookmark add/delete/detail) and a void send() overload to PixivAPIClient
• Decode is_bookmarked from Pixiv illust responses; add BookmarkDetail model for public/private state
• Floating glass heart button in detail view and lightbox: tap to toggle public bookmark, long-press context menu for public/private/remove
• Private bookmarks show a lock badge over the heart
• Grid cells show a heart indicator (bottom-right) for bookmarked posts; page-count pill moved to bottom-left
• Lightbox polish: loading spinner while high-res image loads, fully opaque backdrop, glass effect on X and heart buttons, nav/tab bars hidden during lightbox

Test plan

• Open detail view for an unbookmarked post — heart button is outline, bottom-right
• Tap heart — fills red, API call adds public bookmark
• Tap heart again — unfills, bookmark removed
• Long-press heart — context menu with Bookmark Publicly, Bookmark Privately, Remove Bookmark
• Bookmark privately — heart fills red with gray lock badge
• Navigate to a previously-bookmarked post — heart pre-filled on load
• Grid shows heart badge on bookmarked posts, page-count pill on bottom-left
• Tap image to open lightbox — heart button visible, X button same size, loading spinner shows while image loads
• Nav bar and tab bar hidden during lightbox

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