feat(gui): dashboard layout on the original CyberColors palette

Overview is a row of stat tiles -- small device-class label, the number at
30px in that device's colour, a supporting line and a load bar -- over two
panels holding the process table and what is attached to the machine. The
previous pane spelled every reading as `label: value` at one size and one
weight, so nothing was findable without reading all of it.

The palette is the egui GUI's `CyberColors`, recovered from git rather
than reinvented: GitHub-dark grounds under cyan CPU, green accelerators,
magenta memory, orange disk and blue network, matching the TUI's
conventions. Section titles carry their device colour again. 4.0.0 had
replaced the lot with a neutral palette of my own invention, which is why
the port stopped looking like simon even once it was legible.

Threshold bands are the original four at 90 / 70 / 50 rather than the
three at 85 / 60 the rewrite used, so 55% reads cyan again, not green.

Row values stay neutral until 70%. The tiles are where colour earns its
place; twenty neon percentages in a table is what made the first attempt
at this read as an alarm panel rather than an instrument.

Also fixes a dead `NETWORK` constant that clippy caught: a blanket
neutralise of row colours had flattened every section heading to plain
text, taking the device-class titles with it.

840 tests, 0 failures. clippy -D warnings clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>