feat: add PWA support with Android share intent

Make Favoritter installable as a Progressive Web App with offline
static asset caching and Web Share Target API for Android.

New files:
- internal/handler/pwa.go: handlers for manifest, service worker,
  and share target
- web/static/sw.js: service worker (cache-first static, network-first
  HTML) with {{BASE_PATH}} placeholder for subpath deployments
- web/static/icons/: placeholder PWA icons (192, 512, 512-maskable)

Key design decisions:
- Share target uses GET (not POST) to avoid CSRF token issues — Android
  apps cannot provide CSRF tokens
- Manifest is generated dynamically to inject BasePath into start_url,
  scope, icon paths, and share_target action
- Service worker served at /sw.js with Cache-Control: no-cache and
  BasePath injected via string replacement
- handleShare extracts URLs from Android's "text" field as fallback
  (many apps put the URL there instead of "url")
- handleFaveNew replaced with handleFaveNewPreFill that reads url,
  description, notes from query params (enables share + bookmarklets)
- SW registration in app.js reads base-path from <meta> tag (CSP-safe)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handler/fave.go

@@ -48,19 +48,6 @@ func (h *Handler) handleFaveList(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
-// handleFaveNew shows the form for creating a new fave.
-func (h *Handler) handleFaveNew(w http.ResponseWriter, r *http.Request) {
-	user := middleware.UserFromContext(r.Context())
-
-	h.deps.Renderer.Page(w, r, "fave_form", render.PageData{
-		Title: "Ny favoritt",
-		Data: map[string]any{
-			"IsNew":          true,
-			"DefaultPrivacy": user.DefaultFavePrivacy,
-		},
-	})
-}
-
 // handleFaveCreate processes the form for creating a new fave.
 func (h *Handler) handleFaveCreate(w http.ResponseWriter, r *http.Request) {
 	user := middleware.UserFromContext(r.Context())