feat: add light/dark theme toggle for HTML reports

[Do-yoon]

Summary

Adds a light/dark theme toggle to the flamegraph and table HTML reports.
Closes #650.

What's included

• Toggle button (🌙/☀️) in the navbar
• Initial theme follows the OS prefers-color-scheme
• Selection persisted via localStorage
• data-theme is set before first paint to avoid FOUC
• CSS variables for light and dark palettes (dark uses a black base)
• Bootstrap overrides for navbar, modals, forms, tables, DataTables pagination, search input, dropdowns and scrollbars
• Recolored flame graph bar fills for the dark theme
• News entry (news/650.feature.rst)

Test plan

• Generate flamegraph report and verify both themes
• Generate table report and verify both themes (incl. Previous/Next, search)
• Verify initial paint follows OS dark/light preference
• Verify localStorage persistence across reloads

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.22%. Comparing base (776fe15) to head (0243fa8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #939   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files          99       99           
  Lines       12556    12556           
  Branches      432      432           
=======================================
  Hits        11580    11580           
  Misses        976      976           

Flag	Coverage Δ
cpp	92.22% <ø> (ø)
python_and_cython	92.22% <ø> (ø)

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[sarahmonod]

This PR definitely deserves a NEWS entry. @Do-yoon do you know how to use towncrier? bloomberg/pystack#305 has an example if you want one.

[godlygeek]

@pradyunsg I'm going to defer to you for reviewing this one, if you don't mind doing it.

[Do-yoon]

@sarahmonod

Thanks for pointing me to towncrier — I hadn't used it before, so I really appreciate the pystack#305 example.

Before I push, I just want to make sure I've got the convention right: I'm planning to add a single fragment at news/650.feature.rst containing a one-line description of the feature. Does that look correct to you, or should I use a different filename/category?

[godlygeek]

Yes, that's the correct filename

[Do-yoon]

I've added the news fragment at news/650.feature.rst and pushed it as a new commit, so this should be ready for another look whenever you get the chance. Let me know if anything else is needed.

[Do-yoon]

@godlygeek
@pradyunsg

Rebased onto the latest main to resolve a merge conflict.

The conflict was only in src/memray/reporters/templates/assets/table.js, which is a generated webpack bundle — both this PR (dark theme changes in assets/common.js) and #857 (row deduplication in assets/table.js) had regenerated it, so it couldn't be merged textually. There were no conflicts at the source level.

To resolve it, I took main's version of the bundle and regenerated it with npm ci && npm run build on top of the rebased sources, so the new bundle includes both the deduplication change from #857 and the theme support from this PR. The other bundles didn't need regenerating since #857 doesn't touch common.js.