Add TypeScript migration, image resizing, media upload UX, and multimedia support

- Migrate static JS to TypeScript (static-ts/ → compiled to internal/api/static/)
- Add image resizing on upload: JPEG/PNG/WebP scaled to max 1920px at quality 80
- Extract shared upload logic into upload.go (saveUpload, saveResizedImage, saveResizedWebP)
- Add POST /media endpoint for drag-drop/paste media uploads with markdown ref return
- Add background music player with video/audio coordination (autoplay.ts)
- Add global nav, public feed, hashtags, visibility, Markdown rendering for entries
- Add Dockerfile stage for TypeScript compilation (static-ts-builder)
- Add goldmark, disintegration/imaging, golang.org/x/image dependencies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -6,12 +6,23 @@ RUN npm ci
 COPY webapp/ ./
 RUN npm run build
 
-# Stage 2: Build Go server
+# Stage 2: Compile static TypeScript
+FROM node:22-alpine AS static-ts-builder
+WORKDIR /build/backend
+COPY backend/static-ts/ ./static-ts/
+RUN mkdir -p ./internal/api/static \
+ && cd ./static-ts \
+ && npm ci \
+ && npm run build
+
+# Stage 3: Build Go server
 FROM golang:1.25-alpine AS go-builder
 WORKDIR /app
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
 COPY backend/ ./
+# Inject compiled static JS
+COPY --from=static-ts-builder /build/backend/internal/api/static/ ./internal/api/static/
 RUN go test ./...
 # Inject built SPA into embed path
 COPY --from=webapp-builder /webapp/dist ./internal/api/webapp/