Change 18+ to 19+ and redesign NSFW UI (badge + global toggle)

[realproject7]

Changes

1. Rename 18+ → 19+ everywhere

Some countries (e.g., South Korea) use 19 as the age cutoff. Change all references:

• Filter labels: "18+" → "19+"
• Checkbox text: "This story contains adult content (18+)" → "(19+)"
• NSFW badge text
• Any other user-facing text referencing "18+"

2. Add 19 badge on book cover images

For stories with is_nsfw = true, display a circled "19" badge on the top-right corner of the cover image — same style as Korean webtoon platforms (Toptoon reference).

• Small circular badge, solid background
• Positioned top-right of the StoryCard cover image
• Visible on both discover page cards and story detail page cover

3. Move 19+ toggle to the global nav bar

Replace the current 19+ checkbox in the FilterBar with a toggle button next to the PlotLink logo in the top nav bar. Reference: Toptoon's "19" pill next to the logo.

• Small pill/badge toggle next to "PlotLink" text in NavBar
• Click to toggle NSFW content visibility globally
• Persist preference in localStorage (already implemented)
• Remove the 19+ checkbox from the FilterBar (avoid duplication)
• Toggle state applies to all pages (discover, profile, etc.)

Acceptance Criteria

• All "18+" text changed to "19+" across the codebase
• Circled "19" badge on cover images for NSFW stories
• 19+ toggle pill in NavBar next to PlotLink logo
• Toggle persisted in localStorage
• FilterBar 19+ checkbox removed (replaced by nav toggle)
• StoryEditPanel checkbox text updated to 19+
• Create page checkbox text updated to 19+

[realproject7]

Implemented in PR #1222.