chore(release): v0.3.1 (#260)

First tagged build of the 0.3 line off main. Bumps the version across all
sources (pyproject + frontend package.json + Tauri conf + Cargo + both lock
files + the source-checkout fallback in core/version.py) and adds the [0.3.1]
CHANGELOG section.

Ships:
- #256 — browser/Docker file-export crash (invoke undefined)
- #249 — version surfaced in web/Docker UI + desktop-only updater hidden
- #255 — transcribe stream now surfaces the real ASR/model-load failure

Tagging v0.3.1 triggers release.yml (desktop binaries) and docker.yml
(GHCR :0.3.1 / :0.3 / :latest).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/core/version.py

@@ -12,4 +12,4 @@ from importlib.metadata import PackageNotFoundError, version
 try:
     APP_VERSION = version("omnivoice")
 except PackageNotFoundError:  # non-installed source checkout
-    APP_VERSION = "0.3.0"
+    APP_VERSION = "0.3.1"