[codex] Render admin page with Leptos

[legokichi]

Summary

• render /admin with Leptos SSR instead of Askama
• keep the existing admin POST flows for CSV apply and waypoint deletion
• remove the unused Askama template and dependency
• remove the Askama reference from README

Validation

• make test
• make clippy
• make fmt-check
• make build
• make test-e2e
• git diff --check

[github-actions]

Terraform Plan Results

Terraform Plan Output

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration
and found no differences, so no changes are needed.

Pusher: @legokichi, Action: pull_request

[github-actions]

Terraform Plan Results

Terraform Plan Output

No changes. Your infrastructure matches the configuration.

Terraform has compared your real infrastructure against your configuration
and found no differences, so no changes are needed.

Pusher: @legokichi, Action: pull_request