Bump to v3.1.3: single-instance lock + update/read-only reliability fixes

Fixes the chained-fault runaway Andre hit after an update (multiple copies
stacking, audio climbing to deafening, terminal kill to recover).

- Single-instance lock (SingleInstanceCoordinator + SingleInstanceDialog,
  wired in Program.Main). A named mutex makes two copies impossible. A second
  launch offers: switch to the running copy (default; surfaces it from the
  tray via a named activation event), or force the running copy closed and
  start fresh (Process.Kill, retried elevated if the target is elevated).
  This is the structural fix that makes the stacking runaway impossible.
- Prompt-free update exit. InstallUpdateAsync sets updatingInProgress before
  Application.Exit(); the close path's skipPrompt now honours it, so no
  unsaved-changes dialog (whose default button is Cancel) can abort the
  update's restart.
- Read-only persistence fix. BuildCurrentProfile now carries
  currentProfileReadOnly into the saved snapshot. Previously a deliberate
  save of a locked profile wrote ReadOnly=false, silently unlocking it on
  disk — which re-armed the save prompt that then blocked the update.
- In-process double-install guard. updateInstallStarted stops the ~4 s
  startup check and the background poll both staging an install + helper.
- Docs: About box, RELEASE_NOTES.md, readme.html + MANUAL.md ("Only one copy
  of RemSound runs at a time").

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

MANUAL.md

@@ -208,6 +208,16 @@ Item| Shortcut| What it does
 
 **Keyboard access:** the tray icon is reachable through standard Windows shortcuts — **Windows + B** moves focus to the notification area, arrow keys navigate, Enter activates, and the application context-menu key (or Shift+F10) opens the right-click menu without a mouse.
 
+### Only one copy of RemSound runs at a time
+
+RemSound only ever runs as a single copy. If you try to open it while it's already running — for example by double-clicking it when it's already sitting in the system tray — it won't start a second one. Instead it asks what you'd like to do:
+
+  * **Switch to the running copy** — brings the copy that's already running back to the front. This is the default and usually what you want. Remember it may be minimised to the system tray, down by the clock.
+  * **Force the running copy to close and start fresh** — only needed if the running copy is stuck or not responding. It closes that copy and starts a new one. If the stuck copy was in the middle of a recording, that recording is lost — so this is the “get me out of trouble” option, not an everyday one.
+  * **Cancel** — do nothing.
+
+
+
 ## 5. Menus (File, Record, Options, Help)
 
 There are four menus on the main window: **File (Alt+F)** , **Record (Alt+K)** , **Options (Alt+O)** and **Help (Alt+H)**. The Record menu opens with Alt+K rather than Alt+R because Alt+R is already used by the **Receive audio** checkbox on the main window. The menu's title is shown as “Record (Alt+K)” so you can find the shortcut even though there is no K in the word.