style(project): format Go with gofmt, set up Prettier for the frontend

Run gofmt -w across the backend, normalising the manually-aligned := blocks
to the gofmt standard. No code behaviour changes.

Add Prettier (+ prettier-plugin-svelte) to the frontend with the SvelteKit
default config (tabs, single quotes) so formatting is reproducible, then run
it over the whole tree. Add format / format:check npm scripts and a
.prettierignore (build output, generated schema.ts, static assets).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/storage/disk.go

@@ -5,12 +5,12 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	_ "golang.org/x/image/webp" // register WebP decoder
 	"image"
 	"image/color"
+	_ "image/gif" // register GIF decoder
 	"image/jpeg"
-	_ "image/gif"                    // register GIF decoder
-	_ "image/png"                    // register PNG decoder
-	_ "golang.org/x/image/webp"      // register WebP decoder
+	_ "image/png" // register PNG decoder
 	"io"
 	"os"
 	"os/exec"