Cleanup: reorganize components and composables dirs

[conservationtimothy]

Goal

Organize components/shared and the root composables directory into subdirectories, and move the related tests. Closes #680

Screenshots

None.

What I changed and why

• Moved files in components/shared and the root composables directory into subdirectories grouped by domain
• Shared components now sit in media, map, downloads, header, toast, filters, and sidebar
• Composables now sit in alerts, auth, config, downloads, filters, map, media, and toast
• Seven shared components and two composables remain in their parent directories as they do not belong to a single group
• Added composables/** to imports.dirs so nested composables stay auto-imported without explicit imports
• All import paths and co-located unit tests updated to match the new locations

How I convinced myself this is right

The tests pass and the UI still renders.

What I'm not doing here

I did not rename components, change page behaviour. I did not move AdminConfigGear, DataLoadError, EmptyStateIllustration, GlassCard, SearchBar, Tooltip, ViewTypePill, useRecordCache, or useViewType.

LLM use disclosure

GPT Sol 5.6