Fix missing Linux AppImage and launcher icons

[bcbetterninja]

The Linux AppImage shipped a logo but omitted .DirIcon and an icon-theme entry. The user-level launcher also referenced Icon=BetterMail without installing that icon, leaving launcher/dock icons unresolved.

Include PNG .DirIcon metadata in both AppImage packaging paths and the hicolor icon in release packages. On AppImage startup, install a persistent user icon and bettermail.desktop under XDG_DATA_HOME (or ~/.local/share), pointing to the current AppImage path. Reuse this registration when choosing the default mail app, and match the X11 window class to StartupWMClass. Ordinary startup does not change default mail associations. Registration failures do not prevent startup.

The launcher uses an absolute persistent icon path, supports moved AppImages and escaped filenames, and avoids temporary mount paths. File-manager thumbnails require AppImage support in the host file manager; .DirIcon now supplies the expected metadata (AppDir specification).

Validation:

• Solution build: zero warnings/errors; all 557 tests passed.
• New tests cover persistent PNG installation, moved AppImage registration, URI schemes, filename escaping, and invalid paths.
• Repacked an AppDir with the configured Velopack tool and extracted the resulting AppImage: .DirIcon, root logo, hicolor icon, and desktop metadata verified.
• Real GLib DesktopAppInfo check resolved the installed icon and launched a synthetic executable whose name contains spaces, dollar/backtick/quote/percent/backslash characters.
• Bash packaging syntax and git diff whitespace checks passed.

[coderabbitai]

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