fix(reader): render pages at a consistent width instead of thin stripes

Reader page sizing was height-driven — `max-height: 90vh` in single mode
and unbounded natural width in continuous mode. A long vertical page hit
the height cap first and, preserving aspect ratio, collapsed into a thin
vertical stripe; narrow and wide scans rendered at different widths.

Switch both modes to width-driven sizing via a capped `--reader-page-width`
reading column (min(100%, 700px)): every page renders at the same width
regardless of intrinsic dimensions, and tall pages extend downward and
scroll instead of shrinking. Top-align the single-mode grid and make the
prev/next chevrons sticky so they stay reachable on a long page.

Add a Playwright spec asserting equal rendered width across differently-
proportioned pages and that a tall page exceeds the viewport height. The
page-context-menu fixture served a 1x1 image that now renders as a 700x700
square taller than the viewport, making Playwright scroll it into view
before right-clicking — a synthetic scroll that trips the menu's by-design
close-on-scroll. Give that fixture a realistic landscape aspect so the page
fits the viewport, matching real user behaviour (no scroll on right-click).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.94.0",
+  "version": "0.94.1",
   "private": true,
   "type": "module",
   "scripts": {