Bump to v1.4.0: recording-settings dialog cleanup, menu mnemonics

Ships the dialog reorganisation and mnemonic adjustments from
commit 45b709d. No wire-format or audio-pipeline changes — v1.3 and
v1.4 peers interoperate.

About dialog gets a v1.4 release-notes block; csproj Version
property bumped 1.3.0 → 1.4.0; RELEASE_NOTES.md rewritten with the
v1.4 highlights and a one-paragraph upgrading-from-v1.3 note (clean
auto-update via Help → Check for updates is now expected to work on
Dropbox-installed copies since v1.3's helper hardening took effect).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/RemSound.App/RemSound.App.csproj

@@ -14,7 +14,7 @@
          tag_name on the latest GitHub release; bump it on every public release. The
          AssemblyVersion / FileVersion default to this value, and Assembly.GetName().Version
          is what the About dialog and the updater both read. -->
-    <Version>1.3.0</Version>
+    <Version>1.4.0</Version>
   </PropertyGroup>
 
   <ItemGroup>