feat: implement shared-album multi-user photo album

Backend (Go/Gin/pgx): env config, pgx pool with retries, Garage S3 client
(path-style, region garage), embedded golang-migrate migrations, JWT auth with
refresh rotation, albums CRUD, members/roles/invites with no-escalation matrix,
photo upload + async imaging/goexif WebP variants, cursor pagination, presigned
URLs, likes/comments, SSE hub (album + personal), notifications, share links.

Frontend (SvelteKit 2 + Svelte 5 + Tailwind v4): API wrapper with 401 refresh,
auth store, all screens, Uploader/PhotoGrid/Lightbox/Comments, SSE client with
reconnect + REST catch-up, three themes + system via data-theme.

Deploy without Docker: Makefile builds the frontend into backend/static and
embeds it (go:embed) into a single statically linked binary; systemd unit and
nginx SSE example included.

Use gen2brain/webp (-tags nodynamic) instead of cgo kolesa-team/go-webp to keep
CGO_ENABLED=0 static builds; design.md/proposal.md updated accordingly.

Makefile

@@ -0,0 +1,31 @@
+.PHONY: build frontend backend run migrate tidy test clean
+
+frontend:
+	cd frontend && npm ci && npm run build
+	rm -rf backend/static/*
+	cp -r frontend/build/* backend/static/
+
+backend:
+	mkdir -p bin
+	# -tags nodynamic forces gen2brain/webp to use its pure-Go transpiled path
+	# (no purego/dlopen), keeping the binary fully static.
+	cd backend && CGO_ENABLED=0 go build -tags nodynamic -trimpath -ldflags="-s -w" \
+		-o ../bin/shared-album ./cmd/server
+
+build: frontend backend
+
+run: backend
+	cd backend && ../bin/shared-album
+
+migrate:
+	cd backend && go run ./cmd/migrate
+
+tidy:
+	cd backend && go mod tidy
+
+test:
+	cd backend && go test ./...
+
+clean:
+	rm -rf bin backend/static/*
+	touch backend/static/.gitkeep