improve avalonia consistencies

[Gabriel Dufresne (GabrielDuf)]

This pull request introduces a new, reusable LogTextEditor control based on AvaloniaEdit to provide a performant, colored, and selectable log/console viewer across the app. It replaces the previous use of SelectableTextBlock for log output in dialogs, improving performance, visual consistency, and theming. The update also includes some UI tweaks for consistency and polish. (#4884)

Logs & command output

• Replaced the per-line SelectableTextBlock log rendering with a shared LogTextEditor control (read-only AvaloniaEdit.TextEditor). This gives all log/output views virtualized scrolling, free character-level selection across lines, per-line severity colors, and a theme-aware hyperlink color.
• Applied to the UniGetUI log / Manager logs / Operation history pages (via BaseLogPage) and to the operation output and failed-operation dialogs.
• Added the Avalonia.AvaloniaEdit dependency and registered its theme in App.axaml.

Settings pages

• Page title now aligns with the left edge of the settings tiles (back button moved into the gutter).
• Navigation tile chevron switched from a text glyph to the forward.svg icon so it's vertically centered and theme-correct.
• System-tray icon style tile no longer renders as a standalone (rounded) tile mid-group.
• "View logs" card defaults to a standalone rounded tile (Pip still groups it with the alias warning).

Package list (abstract view)

• Filter pane: filled magnifier icon for Search mode (matches Sources/Filter), symmetric left/right padding, rounded
  corners, and removed the one-off box shadow.
• Grid cards: taller cards so the checkbox and ⋯ overflow button stack without overlapping/clipping; sized and aligned the overflow button.
• Icons cards: checkbox/⋯ row sized so the checkbox isn't clipped and both are aligned and centered.

Webview pages (Help / Release notes)

• "View page on browser" button is flat/transparent like the toolbar.
• Help nav buttons (back/forward/home/reload) match the main toolbar button, with a faded (not gray) disabled state and theme-colored icons.
• Added bottom spacing between the toolbar and the webview.