Testing-feedback fixes: tab order, label wording, parametric editing, recording default

* Connectivity tab order: Add peer by IP now sits right after Rename peer (before the Discovered
   list), grouped with the connected-peer actions, per Ed's requested order. Dropped the second
   section header (only the lock toggle was under it).
 * Audio I/O labels: "Set volume for all received audio" → "Master volume for received audio" (the
   code never got this rename, only the manual had). Device lists now say "audio": "...for received
   audio" and "WASAPI/ASIO audio inputs/outputs to send". Labels + AccessibleNames updated together.
 * Add EQ band dialog: the spin/edit boxes now select-all on focus, so a typed value REPLACES what's
   there instead of being inserted next to it and reverting (typing 2.5 over 4.0 now works).
 * Parametric bands list: Left/Right arrow nudge the selected band's gain by half a dB, live —
   up/down still move between bands. NVDA re-reads the band's new dB.
 * Recording settings: source list reordered to Both (top, now the default) / Received / Sent;
   display order decoupled from the RecordingSource enum. Default RecordingSettings.Source = Both.
 * Preferences: Colour theme is now first in the General tab order.

Manual updated. Build clean; --selftest passes; deployed to both test folders. Held for next release.

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

src/RemSound.Core/AppConfig.cs

@@ -75,6 +75,11 @@ public sealed class AppConfig
     /// <see cref="Profile.PeerShaping"/>.</summary>
     public bool ShowPanEqTab { get; set; } = true;
 
+    /// <summary>Which colour theme the window uses: "system" (follow Windows light/dark — the default),
+    /// "light", or "dark". Read once at startup by Program.Main and applied via Application.SetColorMode;
+    /// changing it takes effect on the next launch. Machine-wide, purely visual.</summary>
+    public string ThemeMode { get; set; } = "system";
+
     /// <summary>The machine-wide "named peers" book — peers the user has deliberately renamed, keyed by
     /// the peer's stable identity (machine name, or address for a nameless manual peer). A name applies in
     /// every profile and shows wherever that peer appears. Managed via the Connectivity tab's Rename peer