feat(invoice): add fee alerts and role-aware summaries

[Oryke]

Summary:

• Add Stellar fee spike polling and a dismissible invoice-page warning banner.
• Add local-time relative timestamps across common activity/history/comment displays.
• Gate invoice detail actions by creator/recipient/viewer role and reject unauthorized split metadata PATCH requests.
• Add split summary tables to private invoice detail and public verification pages.

Closes #510
Closes #511
Closes #512
Closes #513

Validation:

• Passed: npx eslint on touched files.
• Passed: git diff --check.
• Blocked: npm test fails before running tests because @rolldown/binding-linux-x64-gnu is missing from node_modules.
• Blocked: npm run build fails on pre-existing parse/import errors in src/app/dashboard/page.tsx, src/app/layout.tsx, src/components/Skeleton.tsx, src/components/WalletConnect.tsx, plus a duplicate import that was fixed in touched src/app/invoice/new/page.tsx.

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