Feature: top-centre hide-until-hover Top Bar theme and 5% low-usage alarm #163

Problem

The taskbar widget can be hard to place: on a crowded taskbar or a multi-monitor setup there is often no good spot for it, and checking usage means looking down at the taskbar. There is also no warning when a limit is about to run out, so it is easy to hit a cap mid-task.

Proposed improvement

1. An optional Top Bar built-in theme: a wide, short strip docked at the top centre of a chosen monitor. It stays collapsed as a small click-through tab and slides down when the cursor rests on it. One block per enabled provider, left to right (new providers appear on the right), each limit with %, a thin bar and a "Resets in 2h 14m" countdown. Follows Windows light/dark.
2. A general hide-until-hover placement option in the theme engine (any theme docked to a monitor's top or bottom edge can use it), with Settings > Display controls.
3. A 5% low-usage alarm: when a headline window (five-hour, weekly, monthly) or a Claude model cap of any enabled provider/account reaches 5% remaining, play a short sound once (at most once a minute), keep edge-docked surfaces revealed, and offer Snooze 1h.

Screenshots:

Alternatives considered

A floating custom theme placed at the top of the screen works, but it stays visible over other windows and cannot hide or warn. A separate overlay app reading usage-cache.json was prototyped first; building it into the theme engine avoids a second process and reuses the existing providers, accounts and dashboard.

Additional context (optional)

An implementation is ready in a PR (linked below). CONTRIBUTING asks to discuss substantial changes first, so this issue is here for that discussion; I'm happy to split the PR (placement, theme, alarm) or change the approach.