chore(release): v0.8.3 version bump + changelog (#46)

* chore(release): bump 0.8.2 -> 0.8.3

Version bumped across desktop/package.json, Cargo.toml (+ only the `gridline`
entry in Cargo.lock — the sqlx family is untouched), tauri.conf.json, both
landing-page `const version` strings, the README's two download tables and
'current release' banners, the MAINTENANCE comment, and the version tests.

Adds the 0.8.3 changelog entry and README release note covering the external
client-tool resolution fix (issue #44), the MariaDB client-option fix, the four
restyled tool pages, and the app-icon revert.

Historical references (older changelog entries, '(v0.8.2)' feature notes) are
deliberately left alone.

* test(ui): scope CopyButton's unhandled-rejection capture to its own error

The listener is process-global and Vitest runs several test files per worker, so
an unrelated file's rejection could be recorded and fail this test. Filter the
capture to the error this test creates, and stop the listener in a finally so a
failing assertion cannot leak it into later tests.

Verified non-vacuous: against a CopyButton without the try/catch both 'stays
quiet' tests still fail with the expected assertion.

desktop/src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Gridline",
-  "version": "0.8.2",
+  "version": "0.8.3",
   "identifier": "com.adrianbonpin.gridline",
   "build": {
     "beforeDevCommand": "bun run dev",