Add pagination to proposal list

[ManhND226677]

Summary

• add pagination controls to the proposals list
• show 20 filtered proposals per page
• defer IPFS title loading and active-user vote lookups to only the proposals visible on the current page

Fixes #138.

Verification

• npm run lint -- --fix-dry-run (passes with existing repository warnings)
• npx eslint --ext .vue pages/proposals/index.vue (passes with two existing-style console warnings)
• git diff --check
• npm run build (fails in this local environment because node-sass@4.14.1 does not support Node runtime 137; this repository declares Node 14.21.3/npm 6.14.11. The local install also reports unrelated simplemde/codemirror and marked bundling errors.)