Add cartoon reader rendering for image-led PlotToon content

[realproject7]

Context

PlotToon publishes cartoon/webtoon plots as markdown image sequences (one image per cut). StoryContent already allows sanitized markdown images, but the reader UI is prose-first.

Related PlotToon planning: realproject7/plottoon#50 and realproject7/plottoon#42.

Proposed scope

Conditional rendering via storyline.content_type

• Story detail page fetches storyline data which includes content_type (after Add content_type support for fiction vs cartoon stories #1212)
• Pass contentType prop to StoryContent component
• When contentType === "cartoon": apply vertical scroll webtoon layout
• When contentType === "fiction" (or undefined): keep current prose styling

Cartoon reader CSS

• Full-width responsive images, centered
• No prose line-height/paragraph spacing between images (images should tile vertically with minimal gap)
• Mobile-optimized: images fill viewport width
• Dark or neutral background behind image sequence (optional, common in webtoon readers)

Badges

• Add "Cartoon" badge on story cards (same style as "AI WRITER" badge)
• Show on discover page, profile page, and story detail page

What stays the same

• Markdown sanitization unchanged (img tags already allowed with src, alt, title)
• IPFS-only URL expectation unchanged
• Fiction plots retain current prose styling completely

Depends on

• Add content_type support for fiction vs cartoon stories #1212 (content_type field must exist)

Acceptance criteria

• Cartoon plots display as vertical webtoon image sequence
• Fiction plots unchanged
• Cartoon badge visible on story cards
• Markdown images remain sanitized
• Responsive on mobile
• Visual check covers at least one cartoon plot with multiple images