Bump to v1.8.0: updater hardening + plain-language manual rewrite

Updater (RemSoundUpdater.cs), all from Andre's feedback:
* CheckForUpdateAsync now requests /releases?per_page=100 instead of the
  default 30-item page, so a burst of server-vX.Y relay releases can't
  push the newest client release off page 1.
* The install helper's robocopy now excludes remsound.config.json and the
  logs / profiles / recordings folders — an update replaces app files
  only and can never overwrite the user's own config or data.
* On a successful update the helper now also deletes _update-helper.log
  and any stale update-failed.txt (the _update folder was already
  removed), leaving a tidy install folder. The failure branch still keeps
  them all for diagnosis.
* update-failed.txt rewritten as plain user-facing instructions: numbered
  steps, no brand names, names the real _update folder, no robocopy
  jargon. The exit code now goes to _update-helper.log only.

Manual (readme.html): rewritten in plain language — developer jargon
removed or explained in everyday terms — and a second pass removed the
keystroke-navigation choreography and screen-reader narration from the
prose. The Keyboard shortcuts section and all shortcut reference are
kept intact.

No wire-format or audio-pipeline changes — v1.5 through v1.8 interoperate.

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

src/RemSound.App/AboutDialog.cs

@@ -20,6 +20,28 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
+        RemSound v1.8
+
+        Updater polish and a rewritten user manual. No
+        wire-format or audio-pipeline changes — v1.5 through
+        v1.8 peers interoperate.
+
+        Changes:
+          * Check for updates looks further down the release
+            list, so it reliably finds the newest RemSound
+            version.
+          * An update can no longer overwrite your own
+            settings, profiles, logs or recordings — it only
+            replaces program files.
+          * After a successful update the install folder is
+            left tidy: temporary update files, the update log
+            and any old failure note are cleared automatically.
+          * If an update ever fails, the note it leaves
+            (update-failed.txt) is now written in plain
+            language with clear steps to follow.
+          * The user manual (press F1) has been rewritten in
+            plain language throughout.
+
         RemSound v1.7
 
         Updater fix. No wire-format or audio-pipeline changes —