fix(reader): revert layering contract to per-box z-index stacking (#245)

Restores z-index:11 on the box and its panel and drops the
positionedLine z-index lift from f6b2aef.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: Gnathonic

src/lib/components/Reader/TextBoxes.svelte

@@ -609,6 +609,7 @@
     font-family: 'Noto Sans JP', sans-serif;
     /* Word wrapping controlled dynamically by JavaScript */
     border: 1px solid rgba(0, 0, 0, 0);
+    z-index: 11;
     user-select: text;
     -webkit-user-select: text;
     -moz-user-select: text;
@@ -632,6 +633,7 @@
     background-color: rgb(255, 255, 255);
     font-weight: var(--bold);
     font-family: 'Noto Sans JP', sans-serif;
+    z-index: 11;
     user-select: text;
     -webkit-user-select: text;
     -moz-user-select: text;
@@ -666,29 +668,17 @@
     white-space: nowrap;
   }
 
-  /* Per-line mode: each line is placed at its detected quad with a
-     geometry-derived font size. line-height 1 keeps the column/row no
-     thicker than the font size; letter-spacing 0 because the print's
-     tracking is already baked into the quad length the size was fitted to. */
+  /* Original mode with lines_coords: each line is placed at its detected quad
+     with a geometry-derived font size. line-height 1 keeps the column/row no
+     thicker than the font size; letter-spacing 0 because the print's tracking
+     is already baked into the quad length the size was fitted to. */
   .textBox.perLine .ocr-line.positionedLine {
     position: absolute;
     line-height: 1;
     letter-spacing: 0;
     white-space: nowrap;
   }
 
-  /* Layering contract for overlapping blocks (detector sometimes assigns a
-     column to the neighboring block, so boxes overlap): every box's white
-     panel must paint below every box's per-line text. The boxes stay
-     z-index:auto (no own stacking context — tree order still stacks smaller
-     boxes over bigger ones), while positioned line spans get a positive
-     z-index that lifts them above all panels in the page's context. The
-     whole-box panel is kept because it masks messy/angled print that
-     per-line strips cannot cover. */
-  .textBox.perLine .ocr-line.positionedLine {
-    z-index: 1;
-  }
-
   /* A quad that captured multiple print columns (base text + furigana):
      the text flows inside the full quad bbox at the block's reference size,
      wrapping into columns/rows instead of shrinking onto one line. */