[Fix] Remove 19+ toggle from NavBar, restore FilterBar checkbox

[realproject7]

Problem

The 19+ round icon toggle in the NavBar (from #1222 + #1224) doesn't work as intended. Need to safely remove it and restore the original filter behavior.

What to revert

Remove from NavBar

• Remove the 19+ circle toggle icon from src/components/NavBar.tsx (added by Rename 18+ to 19+ and redesign NSFW UI (#1209) #1222, redesigned by [#1224] Redesign 19+ toggle as round circle icon next to logo #1225)
• Remove useNsfwPreference import from NavBar
• NavBar should go back to just: logo + nav links + connect wallet

Restore in FilterBar

• Restore the 19+ checkbox in src/components/FilterBar.tsx that was removed by Rename 18+ to 19+ and redesign NSFW UI (#1209) #1222
• The checkbox was part of the desktop filter row, next to Genre/Language dropdowns
• It should use the "19+" label (not "18+" — keep the rename from Rename 18+ to 19+ and redesign NSFW UI (#1209) #1222)
• It should use the existing URL param approach (?nsfw=1) that FilterBar already used before Rename 18+ to 19+ and redesign NSFW UI (#1209) #1222

What to KEEP (do not revert)

These changes from #1222 are correct and should stay:

• ✅ All "18+" → "19+" text renames (create page, terms, story page, etc.)
• ✅ NsfwBadge component on StoryCard (circled "19" on cover images)
• ✅ useNsfwPreference hook (can stay in codebase, just remove from NavBar)
• ✅ Profile page NSFW filtering via useNsfwPreference (StoriesTab, PortfolioTab)
• ✅ is_nsfw badge on story detail page

Reference: pre-#1222 FilterBar state

Before #1222, the FilterBar desktop row had:

[Sort tabs]  [Writer filter]  [Genre dropdown]  [Language dropdown]  [19+ checkbox]  [X stories]

The 19+ checkbox was a <label> with <input type="checkbox"> that called navigate({ ..., nsfw: e.target.checked }) to add/remove ?nsfw=1 from the URL.

Restore this exact pattern with "19+" label text.

Acceptance criteria

• 19+ toggle removed from NavBar
• 19+ checkbox restored in FilterBar (desktop filter row)
• Checkbox uses "19+" label (not "18+")
• Checkbox toggles ?nsfw=1 URL param (existing behavior)
• NsfwBadge on story cards preserved
• All other "19+" text renames preserved
• Profile page NSFW filtering preserved
• Mobile filter sheet includes 19+ toggle (if it did before)

Refs: #1209, #1222, #1224, PR #1222, PR #1225