docs: design the network and slackware widgets

Two new cards and three fixes to existing ones.

The network card reads the interface byte counters directly and keeps its
own history, one sample per pixel column, so the chart never interpolates.
Both series share one autoscale: drawn independently, a 200kB/s upload would
look like a 40MB/s download. The public address moves from the old config's
5 minute poll to 30 minutes, since a residential address is stable for days.

The slackware card takes its numbers from a sampler rather than the draw
hook, and stores epochs instead of reconstructed dates. The shell function it
replaces parses ls -l output and hardcodes the year, which breaks every
January and on any file older than six months.

Records two things found while checking real values against the design:
/var/log/packages is a symlink, so counting it without a trailing slash
reports 1 package rather than 2817, and the ChangeLog mtime on this machine
currently reads in the future, so the age arithmetic has to survive a
negative difference.

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

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