feat: add skeleton loading UI for all route segments

Related Issue

N/A

Changes

• Add loading.tsx files with animate-pulse skeleton UI for all 6 data-fetching route segments
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• Each skeleton matches the actual page layout to minimize CLS (Cumulative Layout Shift)
• No i18n strings needed — skeletons are purely visual placeholder blocks

How to Verify

1. npm run dev
2. Navigate to any page — skeleton should flash briefly before content loads
3. To simulate slow network: open DevTools → Network → throttle to "Slow 3G", then navigate between pages
4. Verify each skeleton matches the layout of its corresponding page

Checklist

• npm run format:check passes
• npm run lint passes
• npm run typecheck passes
• npm run build passes
• PR title follows Conventional Commits (feat:, fix:, chore:, refactor:, docs:, test:, )