[Fix] 19+ toggle: redesign as round red icon next to logo (reference-based)

[realproject7]

Problem

PR #1222 implemented the 19+ toggle but the design doesn't match the intended style. Needs a follow-up fix.

What to fix

1. Toggle design: round red "19" icon

The 19+ toggle must be a circular red icon with "19" text inside — not a text pill or checkbox.

• Red filled circle with white "19" text
• Small size (~20-24px diameter), matching the reference images
• When off (default): muted/gray circle with "19"
• When on: red circle with white "19" (active state)
• Click toggles between states

2. Location: right next to PlotLink logo

The icon must sit immediately to the right of the "PlotLink" logo text in the NavBar — not in the filter area, not floating elsewhere.

Layout: [P logo] PlotLink [19 circle icon]

3. Remove current toggle implementation

Remove whatever toggle was added in #1222 (NavBar pill or FilterBar element) and replace with this design.

Reference images

The agent must check these local reference images for the exact visual style:

• /Users/cho/Projects/z-design/ref-items/19plus-icon-1.png — toggle OFF state
• /Users/cho/Projects/z-design/ref-items/19plus-icon-2.png — toggle ON state (red, active)

The reference shows a round red circled "19" icon positioned right next to the site logo. Copy this exact pattern.

Acceptance criteria

• Round circular "19" icon (not text pill, not checkbox)
• Red fill + white text when active, muted/gray when inactive
• Positioned immediately right of "PlotLink" logo text in NavBar
• Click toggles NSFW content visibility
• Preference persisted in localStorage
• Previous Rename 18+ to 19+ and redesign NSFW UI (#1209) #1222 toggle removed (no duplication)
• Works on both desktop and mobile nav

Refs: #1209, PR #1222