Support Arduino-style labeled multi-value plotter data

[makermelissa-piclaw]

Fixes #457.

The Arduino IDE Serial Plotter accepts a label:value format with
tab/comma/space separators (and supports many series at once). The
web-editor plotter previously only understood plain CSV / tuple /
list lines, so a sketch that prints Temp:23.4\tHum:55.1 would
either drop the line or only plot the digits it could pull from the
front of the token.

This PR teaches plotValues the labeled format:

• New parseLabeledValues helper splits on ,, \t, or runs of
  whitespace, then peels off an optional label: prefix per token.
• Datasets are now tracked by label when one is provided, so series
  stay on the same line across frames even if the sketch reorders
  them (or omits some on a given frame). Unlabeled lines keep the
  existing positional behavior, so old CSV / tuple / list sketches
  are unaffected.
• When a labeled series doesn't appear on a given frame, that
  dataset is padded with null so the x-axis stays aligned.
• Chart legend is enabled so the labels actually show up.
• Default color palette expanded from 3 to 16 distinct colors and
  reused round-robin, so multi-channel sketches (e.g. the AS7343
  spectral example referenced in the issue) don't all collapse to
  black after the fourth series.

Manual test cases

I unit-tested the parser against the following inputs (all parse to
the expected {label, value} pairs):

• 1,2,3
• 1\t2\t3
• 1 2 3
• Temp:23.4,Hum:55.1
• 405nm_F1:123\t425nm_F2:456\tClear:789
• A:1\tB:-2.5\tC:3e2
• 1,Temp:42,3 (mixed labeled + positional)
• A:1\tB:2\t (trailing tab)

Existing tuple (1,2,3) and list [1,2,3] paths are unchanged.

npm run build passes locally.