Fix DOM-XSS in theme dropdowns via innerHTML concat#188
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Theme names from the database were concatenated into an HTML string
and assigned to innerHTML, so a theme name containing quotes or a
<script> tag could break out of the value attribute and execute as
DOM-based XSS.
Replace the string-concat pattern with DOM-API construction
(document.createElement('option') + option.value = theme) in all four
templates that render the theme datalist: index.html, suggest.html,
write_and_index.html, web_commentary.html. Setting .value directly
leaves the browser to handle attribute escaping and never goes through
an HTML parser.
Add tests/test_theme_xss.py as a regression guard: it greps each of
the four templates for the vulnerable patterns (+ '<option', and
.innerHTML = str) and for the corrected createElement('option') call.
Any future re-introduction of the unsafe pattern will fail CI.
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innerHTML, enabling DOM-based XSS via a malicious theme namedocument.createElement('option')+option.value = theme) in all four affected templates:index.html,suggest.html,write_and_index.html,web_commentary.htmlcreateElementcallCloses #163