feat: add hashfs for static asset cache busting and live clock

- Add assets package with dev/prod build tags
- Dev: serve from filesystem with Cache-Control: no-store
- Prod: use hashfs for cache-busting URLs
- Add LiveClock component to show SSE connection status
- Update templates to use StaticPath for asset URLs

go.sum

@@ -136,6 +136,8 @@ github.com/aymanbagabas/go-udiff v0.3.1/go.mod h1:G0fsKmG+P6ylD0r6N/KgQD/nWzgfnl
 github.com/aymerick/douceur v0.2.0 h1:Mv+mAeH1Q+n9Fr+oyamOlAkUNPWPlA8PPGR0QAaYuPk=
 github.com/aymerick/douceur v0.2.0/go.mod h1:wlT5vV2O3h55X9m7iVYN0TBM0NH/MmbLnd30/FjWUq4=
 github.com/benbjohnson/clock v1.1.0/go.mod h1:J11/hYXuz8f4ySSvYwY0FKfm+ezbsZBKZxNJlLklBHA=
+github.com/benbjohnson/hashfs v0.2.2 h1:vFZtksphM5LcnMRFctj49jCUkCc7wp3NP6INyfjkse4=
+github.com/benbjohnson/hashfs v0.2.2/go.mod h1:7OMXaMVo1YkfiIPxKrl7OXkUTUgWjmsAKyR+E6xDIRM=
 github.com/bep/clocks v0.5.0 h1:hhvKVGLPQWRVsBP/UB7ErrHYIO42gINVbvqxvYTPVps=
 github.com/bep/clocks v0.5.0/go.mod h1:SUq3q+OOq41y2lRQqH5fsOoxN8GbxSiT6jvoVVLCVhU=
 github.com/bep/debounce v1.2.1 h1:v67fRdBA9UQu2NhLFXrSg0Brw7CexQekrBwDMM8bzeY=