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>

backend/internal/api/router.go

@@ -26,6 +26,7 @@ func NewRouter(
 
 	webUI := NewWebUI(authStore, tpStore, stopStore, journalStore, userStore)
 	journalHandler := NewJournalHandler(journalStore, uploadDir)
+	mediaHandler := NewMediaHandler(uploadDir)
 	authMW := RequireAuth(authStore)
 
 	// Health
@@ -70,6 +71,7 @@ func NewRouter(
 		r.Get("/days", webUI.HandleDaysList)
 		r.Get("/days/redirect", webUI.HandleDaysRedirect)
 		r.Get("/days/{date}", webUI.HandleDayDetail)
+		r.Post("/media", mediaHandler.HandleUpload)
 		r.Post("/entries", journalHandler.HandleCreateEntry)
 		r.Get("/entries/{id}/edit", journalHandler.HandleGetEditEntry)
 		r.Post("/entries/{id}", journalHandler.HandleUpdateEntry)