feat: add centerButton, menuButton, linksContent snippets and bars icon to BottomNavigation

[rossnelson]

Description & motivation 💭

Adds customization hooks to BottomNavigation for consumers that need to override the default mobile nav bar sections. All new props are optional and fall back to existing behavior when not provided, so this remains backward-compatible.

The merged implementation keeps BottomNavigation aligned with the current linksSnippet-based mobile nav API from main while preserving the new customization points introduced in this PR.

New snippet props on BottomNavigation:

• menuButton?: Snippet<[{open, onClick}]> — replaces the default left menu/logo button
• centerButton?: Snippet<[{open, onClick}]> — replaces the default namespace-switcher center section
• linksContent?: Snippet<[{open, closeMenu}]> — replaces the default mobile links panel content

New holocene icon:

• bars — three-line hamburger menu icon, consistent with the filter-lines icon style

Screenshots (if applicable) 📸

N/A

Design Considerations 🎨

No design changes beyond exposing existing mobile nav areas as optional snippet overrides.

Testing 🧪

How was this tested 👻

• Manual testing
• E2E tests added
• Unit tests added

Validated with:

• node_modules/.bin/prettier --check src/lib/components/bottom-nav.svelte
• corepack pnpm check

Attempted targeted Playwright integration coverage for mobile navigation, but the configured web server failed to start in this environment before tests executed.

Steps for others to test: 🚶🏽‍♂️🚶🏽‍♀️

1. Render BottomNavigation without the new snippet props and confirm the default mobile nav behavior is unchanged.
2. Pass menuButton and verify it renders in place of the default left button.
3. Pass centerButton and verify it renders in place of the namespace switcher area.
4. Pass linksContent and verify it renders in place of the default mobile links panel content.
5. Render <Icon name="bars" /> and confirm the hamburger icon appears correctly.

Checklists

Draft Checklist

• Merge conflict resolved against main
• BottomNavigation kept compatible with current linksSnippet API
• Manual QA

Merge Checklist

• Formatting/checks run for affected code
• Merge conflict fully resolved
• Optional manual verification in app

Docs

Any docs updates needed?

No docs updates needed.

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Project	Deployment	Actions	Updated (UTC)
holocene	Ready	Preview, Comment	May 29, 2026 4:11pm

[rossnelson]

@copilot resolve the merge conflicts in this pull request

[CLAassistant]

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✅ rossnelson
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[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in 6a66126.