feat: allow links to have custom icons via url or emoji

[pablaber]

Summary

Adds support for custom link icons via URL or emoji, in addition to the default favicon behavior. Users can now choose between favicon (default), a custom icon URL, or an emoji icon for each link in the config editor.

Changes

• src/types/config.ts — Added iconUrl and iconEmoji optional fields to LinkConfig
• src/screens/HomeScreen/components/iconDisplay.ts — New shared utility that resolves link icon display (emoji > iconUrl > icon > favicon)
• src/screens/HomeScreen/components/LinkItem.tsx — Updated to render emoji icons or image icons using the shared getIconDisplay utility
• src/screens/HomeScreen/components/SearchBar.tsx — Updated search dropdown to show emoji icons alongside image favicons
• src/screens/HomeScreen/HomeScreen.tsx — Updated navigating state to show the correct custom icon (emoji or image)
• src/screens/ConfigEditorScreen/components/EmojiPicker.tsx — New emoji picker component with categorized emojis and search
• src/screens/ConfigEditorScreen/components/LinksTab.tsx — Added icon type selector (Favicon/Icon URL/Emoji) per link with emoji picker integration
• src/App.css — Added styles for emoji icons, emoji picker, and icon selector UI in config editor
• src/test/fixtures.ts — Added mockLinkWithIconUrl and mockLinkWithEmoji test fixtures
• src/screens/HomeScreen/components/LinkItem.test.tsx — Added tests for iconUrl, iconEmoji rendering, and priority ordering

Issue Reference

Closes #42

Testing

All tests pass (87 tests across 11 test files). Lint and build succeed.

npm run lint && npm run test && npm run build

Verification

gh pr checkout PRNUM
npm ci
npm run dev

• Open the app and verify existing links still show favicons by default
• Open Settings > Links tab and verify each link has an icon type dropdown (Favicon/Icon URL/Emoji)
• Change a link's icon type to "Icon URL" and enter a custom icon URL — verify it displays on the home screen
• Change a link's icon type to "Emoji" and pick an emoji from the picker — verify it displays on the home screen
• Verify the emoji picker has categories, search functionality, and closes properly
• Reset a custom icon back to "Favicon" using the X button and verify it falls back to the default favicon
• Verify custom icons appear correctly in the search dropdown
• Verify the navigating transition screen shows the correct custom icon
• Run npm run lint && npm run test && npm run build to verify all checks pass

This PR was automatically generated by an AI agent (Claude Code). Please review carefully before merging.