refactor(branding): rename project to LockScreenWallpaper across repo

Standardize branding by renaming Wechselbild to LockScreenWallpaper.
This updates UI strings, configs, and plugin metadata for consistency.

- Rename branding across UI, docs, and config in the repo
- Update Android UI strings and notification titles to LockScreenWallpaper

src-tauri/tauri.conf.json

@@ -1,9 +1,9 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
-  "productName": "Wechselbild",
+  "productName": "LockScreenWallpaper",
   "version": "0.1.0",
   "identifier": "de.wechselbild.app",
   "build": { "beforeDevCommand": "npm run dev", "devUrl": "http://localhost:1420", "beforeBuildCommand": "npm run build", "frontendDist": "../dist" },
-  "app": { "windows": [{ "title": "Wechselbild", "width": 420, "height": 860, "resizable": true }], "security": { "csp": null } },
+  "app": { "windows": [{ "title": "LockScreenWallpaper", "width": 420, "height": 860, "resizable": true }], "security": { "csp": null } },
   "bundle": { "active": true, "targets": "all", "icon": [] }
 }