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>

web/static/js/app.js

@@ -188,4 +188,11 @@
         var match = document.cookie.match(new RegExp("(^| )" + name + "=([^;]+)"));
         return match ? match[2] : null;
     }
+
+    // Register service worker for PWA support.
+    if ("serviceWorker" in navigator) {
+        var baseMeta = document.querySelector('meta[name="base-path"]');
+        var base = baseMeta ? baseMeta.getAttribute("content") : "";
+        navigator.serviceWorker.register(base + "/sw.js", { scope: base + "/" });
+    }
 })();