Better alerts

[mirdaki]

Right now the only indication the timer finishes is a temporary "toast" style alert. This isn't great if you're focused on something, as you're likely to miss it.

Some ideas:

• Have a small chime play. Should be able to turn off or on (likely possible, but may be annoying)
• Have the panel do some kind of flash/animation to visually indicate it's done (likely built off of work for #1)
• Have the SilverBullet task icon (when using as a PWA) flash or change in some fashion (may not be possible with current API)

[mirdaki]

Supporting alert sound with 7cb4e29

Leaving this open, as their should also be a more prominent visual component