Add dark theme for settings UI

[quangdat99x]

Summary

• Switches the MaterialDesignExtensions base theme from Light to Dark and repalettes the centralized brushes in App.xaml (chrome background, primary/secondary hues) to a dark background with teal accents.
• Pins dark text on the few intentionally light chip controls (hotkey input box, ExtendedButton link buttons, Lookupper banner) so they stay readable against the new dark chrome.
• Adds a .gitignore entry for a local run/ folder (a tidy, ready-to-launch copy of the build output used during manual testing; not meant to be tracked).

Why

Requested UI improvement: give Translumo a dark mode option for its Settings window instead of the current light cyan/mint theme.

Test plan

• Built via dotnet publish Translumo.sln -c Release (net8.0-windows, self-contained win-x64).
• Launched the built app and visually confirmed: sidebar/chrome renders dark, selected nav item highlighted in teal, content area dark, proxy info card readable, language switcher readable, and the white Lookupper banner still shows dark, readable text.
• Other settings tabs (Hotkeys, OCR, Translation, About dialog) were not individually re-verified by click-through in this session — please double check those render correctly, since click-navigation could not be exercised in the automated test environment used.

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