Settings windows: the 13 section-local buttons still have no Alt mnemonics

[erikdarlingdata]

Deferred out of #1847's dialog mnemonic sweep, filed so it does not fall through.

#1847 gave every dialog in Lite and the Darling Viewer Alt mnemonics on its action buttons. Both Settings windows got them on the two footer buttons only (_Save / _Close); their 13 section-local buttons are still bare:

• Lite: Pause Collection, Auto (MCP port), Copy Setup Command, Restore Defaults, Manage Mute Rules..., Send Test Email, Validate Settings, Send Test Notification x3 (Teams / Slack / generic), Edit Selected..., Edit Default..., Apply Default to All
• Viewer: the same set plus Auto (web port), Copy URL, Edit Collector Schedules...

Three reasons it was held back rather than done inline, all of which need a decision rather than more underscores:

1. Three buttons share the identical label "Send Test Notification" (Teams, Slack, generic webhook). They are simultaneously visible in one long scrolling page, so they cannot get distinct access keys without renaming the labels - which is a copy change, not a mnemonic change.
2. Five of these buttons have their Content reassigned from code-behind (PauseResumeButton, TestEmailButton, TestTeamsButton, TestSlackButton, TestGenericButton - "Sending...", "Resume Collection", etc). A mnemonic put in XAML alone silently disappears the first time the button changes state, so every state string has to carry it too, and a state-dependent access key (Alt+P for Pause becoming Alt+R for Resume) is a questionable affordance in the first place.
3. Settings is a long scrolling page rather than a compact dialog, so "the primary action buttons" really is just Save and Close. The section buttons are closer to inline controls.

The collision budget is the real constraint: all 15 buttons live in one access-key scope, and #1847's audit treats a duplicate key as worse than no key (Alt+key degrades from "press this" to "cycle focus").

Whatever is decided applies to BOTH apps - the two Settings windows are near-identical and drift between them is the thing to avoid.

[erikdarlingdata]

The exact code-behind sites that have to change alongside the XAML, so point 2 above is not a scavenger hunt. Each of these assigns a bare label back onto a button, which strips any mnemonic the XAML added the first time the button changes state:

• Lite/Windows/SettingsWindow.xaml.cs lines 165, 172 (Pause/Resume Collection), 958 (Send Test Email), 1155 (Teams), 1186 (Slack), 1247 (generic webhook)
• Darling/PerformanceMonitor.Darling.Viewer/SettingsWindow.xaml.cs lines 293, 300, 315 (Pause/Resume/Pausing/Resuming), 1127 (Send Test Email), 1190 (Teams), 1221 (Slack), 1282 (generic webhook)

The transient "Sending..." / "Pausing..." strings do not need a mnemonic (the button is disabled while they show), but every RESTORED string does, or the key works exactly once per launch.