Soften HTML search highlight colours

[LuisMRaimundo]

Summary

Makes the HTML search-term highlight colours softer and lighter (pale pastels) while
keeping multiple terms distinguishable and text readable. Presentation only — no
change to match-finding, counts, ranking, or exported text content.

Changes (save_results.py)

• Replaced the saturated per-term palette with soft pastels:
  #fff3b0, #d8f3dc, #dbeafe, #fde2e4, #f3e8ff, #ffe5d9, #e0f2fe, #fef9c3.
• --hi CSS variable: light #fff59d -> #fff3b0; dark #4b5563 -> #fef9c3.
• .highlight now pins dark text (--hi-text: #111827) so pastels stay readable in
  both light and dark themes, and replaces the bold + thick 2px underline with a
  subtle border-bottom: 1px solid rgba(0,0,0,0.15).

New colours vs old

	Old (saturated)	New (pastel)
term palette	#fff59d, #ffcc80, #f48fb1, #90caf9, #a5d6a7, #ce93d8, #ffab91, #b0bec5, #ffe082, #c5e1a5, #b39ddb, #ef9a9a, #81c784, #64b5f6, #ffb74d	#fff3b0, #d8f3dc, #dbeafe, #fde2e4, #f3e8ff, #ffe5d9, #e0f2fe, #fef9c3
--hi (light/dark)	#fff59d / #4b5563	#fff3b0 / #fef9c3
emphasis	font-weight 600 + 2px underline	dark text + subtle 1px border-bottom

Tests

• python -m unittest discover -s tests -v -> 31 passed (OK)
• Added tests/test_html_highlight.py: highlight markup present, term text preserved,
  colours are from the pastel set (no legacy saturated colours), multiple terms keep
  distinct colours.

Match logic

Untouched. Only CSS/style classes changed; no changes to how matches are found,
counted, ranked, or to the exported text.

Manual verification

_highlight_context_html('the contract amount was paid in full', ['contract','amount'])
->
... <span class="highlight" style="background-color: #fff3b0;">contract</span> <span class="highlight" style="background-color: #d8f3dc;">amount</span> ...
(distinct pale colours, dark text).