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>

internal/handlers/integration_test.go

@@ -7,9 +7,6 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
-	"os"
-	"path/filepath"
-	"runtime"
 	"strings"
 	"testing"
 
@@ -18,26 +15,8 @@ import (
 	"gitea.leeworks.dev/0xwheatyz/gitea-mobile/internal/middleware"
 )
 
-// TestMain changes the working directory to the project root so that
-// template files can be found by handlers that use relative paths.
-func TestMain(m *testing.M) {
-	// Walk up from this source file to find the project root (where go.mod lives).
-	_, filename, _, _ := runtime.Caller(0)
-	dir := filepath.Dir(filename)
-	for {
-		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
-			break
-		}
-		parent := filepath.Dir(dir)
-		if parent == dir {
-			// Reached filesystem root without finding go.mod; run tests from cwd.
-			break
-		}
-		dir = parent
-	}
-	_ = os.Chdir(dir)
-	os.Exit(m.Run())
-}
+// Note: TestMain is no longer needed because templates and static assets
+// are embedded at compile time via go:embed. Tests work from any directory.
 
 // mockGiteaAPI starts an httptest server that simulates the Gitea API
 // endpoints needed by the handlers. Returns the server and a cleanup func.