refactor: simplify PWA handlers and fix review findings

Address code review findings from reuse, quality, and efficiency agents:

- Cache manifest JSON and service worker JS at init (was rebuilt per
  request with allocations and JSON encoding on every hit)
- Add ImagePathsByUser store method for targeted image cleanup (was
  loading 100k full fave objects just to read image_path)
- Add missing aria-label on privacy toggle in fave_list.html (inline
  copy had drifted from the partial — accessibility bug)
- Fix comment/function name mismatch in pwa.go
- Remove redundant user nil-check in handleShare (requireLogin guards)

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

web/templates/pages/fave_list.html

@@ -36,6 +36,7 @@
                                 hx-target="#privacy-{{.ID}}"
                                 hx-swap="outerHTML"
                                 class="fave-action-btn {{if eq .Privacy "private"}}secondary{{end}}"
+                                aria-label="{{if eq .Privacy "public"}}Gjør privat{{else}}Gjør offentlig{{end}}"
                                 title="{{if eq .Privacy "public"}}Gjør privat{{else}}Gjør offentlig{{end}}"
                             >{{if eq .Privacy "public"}}Offentlig{{else}}Privat{{end}}</button>
                         </span>