v0.5.0-beta: merge-with-key import, prior_author provenance, beta versioning

Merge-with-key: decrypt exported posts using original identity seed, re-create
under current identity with prior_author in BlobHeader for provenance tracking.
Download page restructured with stable (v0.4.4) + beta (v0.5.0-beta) sections.
Version bumped across all crates.

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

crates/tauri-app/tauri.conf.json

@@ -1,6 +1,6 @@
 {
   "productName": "itsgoin",
-  "version": "0.4.4",
+  "version": "0.5.0",
   "identifier": "com.itsgoin.app",
   "build": {
     "frontendDist": "../../frontend",