Fit the window at every size and show the battery once

The page now scrolls between the header and the Activity bar on a computer,
so the bar never covers content, and nothing overflows sideways from the
760 x 560 minimum up. The header gives way as the window narrows: the
wordmark goes, the tabs tighten, and below 960 px the logo, Refresh (R still
works) and the pill's second line go. Phones keep scrolling the window.

The battery shows once, in the header chip. Clicking it opens a menu with
the headset's storage, memory, temperature, Wi-Fi, uptime, SteamOS build and
services; the big battery card is gone from Home, and the VR performance
table no longer repeats battery and temperature.

Heading rows wrap their buttons onto a new line instead of squeezing the
heading, and every drop-down is dark.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/vr-utilities.md

@@ -26,7 +26,7 @@ it twice reuses the existing process.
 | Compositor CPU | OpenVR compositor render CPU milliseconds, not game CPU time. |
 | System CPU | `/proc/stat` busy-time delta across the sample, with guest time counted once and iowait treated as idle. |
 | GPU clock | `3d00000.gpu/cur_freq`, converted from Hz to MHz; frequency is not load. |
-| Hottest sensor / battery | Existing thermal-zone and battery sysfs reads from `frame_status.py`. |
+| Hottest sensor / battery | Existing thermal-zone and battery sysfs reads from `frame_status.py`. In the headset HUD only: the app shows them once, in the battery menu at the top. |
 
 OpenVR uses background application mode, which does not start SteamVR or keep
 it running. This mode also returned live timing in a read-only device probe.