feat: embed templates and static assets with go:embed for distroless containers

Replace all runtime template.ParseFiles() calls with template.ParseFS()
using an embedded filesystem, and serve static assets from an embedded FS
via http.FileServerFS(). This eliminates the need for COPY steps in the
Dockerfile and ensures the binary works with readOnlyRootFilesystem: true.

- Add internal/templates/embed.go exposing templates.FS
- Add static/embed.go exposing static.FS
- Update all handlers to use template.ParseFS(templates.FS, ...)
- Update static file server to use http.FileServerFS(static.FS)
- Remove COPY static/ and COPY internal/templates/ from Dockerfile
- Remove TestMain working directory hack (no longer needed)

Closes leeworks-agents/gitea-mobile#231
Closes leeworks-agents/gitea-mobile#220
Closes leeworks-agents/gitea-mobile#221

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/embed.go

@@ -0,0 +1,10 @@
+// Package static provides embedded static asset files (CSS, JS, icons, etc.).
+package static
+
+import "embed"
+
+// FS contains all static asset files embedded at compile time.
+// Files are served via http.FileServerFS in the handlers package.
+//
+//go:embed *.css *.js *.json *.png
+var FS embed.FS