refactor: streaming loader, hierarchical TOC, parser robustness

Rework the reader around a clean BookInfo interface and lazy chapter loading.

Types/parser:
- BookInfo now splits into loadStructure (cheap: spine refs + TOC tree) and renderChapter (one chapter on demand). EpubEnv carries the parsed archive.
- resolveZipPath collapses ./.. and strips #fragments (normalise does not), so relative hrefs like ../Images/x.jpg resolve; used for chapters and images.
- decodeEntry tolerates encodings: UTF-8/UTF-16 BOMs, Latin-1 fallback instead of throwing on invalid bytes.
- Skip spine items with no manifest entry instead of throwing an error.
- Scan the zip for a .opf if container.xml is missing.
- Image inlining rewrites the src URL string, not the whole <img> tag (scalpel
  re-serialises tags, so a whole-tag replace silently missed self-closing imgs).

Navigation:
- Hierarchical TOC: ncxToc (EPUB 2), navToc (EPUB 3), guideToc (<guide> fallback). Nested scrapers use atDepth 1 to take only direct children.

UI:
- Streaming: render the opened chapter on demand, cache it, preload neighbours so arrow paging is instant with a placeholder that covers uncached jumps.
- Sidebar shows the TOC tree with CSS styling. Clicks scroll to #fragments.
- wrapHtml wraps each body in a full HTML document for reliable WebKit render.

Test: 'svitak-test <book> <idx>' dumps one rendered chapter. cabal test stanza fixed to list all needed modules/deps.

app/State.hs

@@ -1,41 +1,22 @@
 {-# LANGUAGE OverloadedStrings #-}
 
-module State
-  ( initialState,
-    prepareView
-  )
-where
+-- | The mutable application state's initial value. Navigation logic lives in
+-- the UI, which renders chapters lazily into 'stCache'.
+module State (initialState) where
 
-import Types (AppState(..), Chapter(chapterTags, chapterTitle), ChapterView(..), EpubEnv, ChapterIndex(..))
+import qualified Data.Map.Strict as M
+import qualified Data.Text as T
+import Types (AppState (..), ChapterIndex (..))
 
--- Get an element from a list without crashing
-safeGet :: ChapterIndex -> [a] -> Maybe a
-safeGet (ChapterIndex n) xs
-  | n < 0 = Nothing
-  | otherwise = case drop n xs of
-      (x : _) -> Just x
-      [] -> Nothing
-
--- loading template
-initialState :: EpubEnv -> AppState
-initialState env =
+-- | Empty state shown while the book's structure loads in the background.
+initialState :: T.Text -> FilePath -> AppState
+initialState title path =
   AppState
-    { cEnv = env,
-      cIdx = ChapterIndex 0,
-      cSpine = [],
-      bTitle = "Svitak - Loading...",
-      zoomlvl = 1.0,
-      activeTask = Nothing,
-      cAllChapters = [],
-      taskVersion = 0
-    }
-
--- prepare the data for the UI
-prepareView :: AppState -> ChapterIndex -> Maybe ChapterView
-prepareView st index = do
-  chapter <- safeGet index (cAllChapters st)
-  pure $ ChapterView
-    { viewTitle = bTitle st <> " - " <> chapterTitle chapter
-    , viewHtml  = chapterTags chapter
-    , viewIdx   = index
+    { stRefs = [],
+      stToc = [],
+      stCache = M.empty,
+      stIndex = ChapterIndex 0,
+      stZoom = 1.0,
+      stTitle = title,
+      stBookPath = path
     }