fix: deterministic startup order, tray About entry and readable menu icons

[gnacho]

What

• Startup syncs are requested in the configured folder order instead of HashMap iteration order, so the boot backlog runs strictly sequentially, one folder at a time, in a stable order (Startup syncs interleave across accounts/folders instead of running strictly sequential #154).
• New About entry in the tray menu, last position, presenting the same dialog as the window's hamburger menu (Add an About entry to the system tray menu #155).
• Tray menu icons use a theme-neutral fixed gray (#9a9996) so they stay readable on dark themes, the Log entry gets its own Lucide scroll-text glyph (it was a duplicate of Open), and About uses a fixed-color variant instead of a symbolic icon that the SNI host renders black (Tray menu icons render dark on dark themes (fixed #3d3846 stroke), Log icon duplicates Open, About icon uses currentColor #157).

Gate

• 659 tests passed, 1 ignored; cargo fmt and clippy -D warnings clean.
• Deployed and validated on a production GNOME session.

Closes #154
Closes #155
Closes #157