feat(tray): add procedurally-drawn icons to the tray menu rows

Each row in the tray context menu (floating, recording, history,
settings, about, status, docs, updater, quit) now shows a small glyph
so options are distinguishable at a glance. Icons are rasterized from
simple shape primitives at 4x supersample in the new menu_icons.rs
module instead of shipping hand-authored PNGs. The recording row's
icon (green play / red dot) also updates in sync with the existing
tray-icon blink while a session is active.

Closes #178