fix(ui): keep the important text visible on narrow terminals (#38)

Three panes painted two independent pieces of text onto one row with nothing
reserving space between them, so below roughly 105 columns the right-hand piece
was simply drawn over the tail of the left one:

  socktop — host: cachyos-gaming | 🔒✗ TLS | (a: about⏱ 500ms metrics | 2000ms

The width arithmetic used str::len(), a byte count, so the emoji in these strings
overstated their width and left orphaned glyphs at the right edge as well. The
process table had the same problem in a different form: it handed the layout
solver a fixed, over-constrained column set, so a narrow pane crushed the
percentage-sized Name column to nothing while fixed-width PID and Mem % kept
their full width — losing the one field that identifies a process.

Add ui::fit (measure in terminal columns, truncate on character boundaries, pick
the richest wording that fits) and give each pane a priority ladder:

- Header: drop the key hints, then the TLS/token badges, then the
  "socktop — host:" prefix, then the metrics/procs words, and only then shorten
  the hostname. Hostname and intervals are what survive longest.
- CPU pane: drop the "CPU Temp:" label, then the now:/avg: labels, then the
  average, then the temperature's decimal, and the temperature itself last — a
  thermal warning outranks a second decimal place.
- Process table: Name is unconditional; CPU %, then Mem, then PID, then Mem %
  are added as the pane widens, so Mem % is the first to go and Name the last.

Also fixes sort-header clicks, which resolved against a Layout that omitted the
column spacing the Table renders with, so a click landed off by up to four
columns. Covered by a test that clicks each label where it is actually drawn.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Cargo.lock

@@ -2426,6 +2426,7 @@ dependencies = [
  "sysinfo",
  "tempfile",
  "tokio",
+ "unicode-width",
  "url",
 ]