fix: resolve flash of unstyled content in dark mode

[TusharThakur04]

Description

This PR resolves the FOUC by resolving system to dark or light using matchMedia.
Earlier system was directly being applied as set-theme = system

Validation

before :

Screencast.from.2026-04-13.19-26-34.webm

after :

Screencast.from.2026-04-13.19-24-03.webm

Related Issues

n/a

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run node --run test and all tests passed.
• I have check code formatting with node --run format & node --run lint.
• I've covered new added functionality with unit tests if necessary.

[cursor]

PR Summary

Low Risk
Low risk: changes are limited to how the initial data-theme is computed and inlined before first paint; main risk is edge-case behavior differences for users with stored theme preferences.

Overview
Fixes initial-page theme application to avoid dark-mode Flash of Unstyled Content by replacing the hardcoded inline snippet with an injected themeScript.

Adds ui/theme-script.mjs, which resolves the stored theme preference (including system) to a concrete dark/light value via matchMedia and applies it to documentElement before render; processing.mjs now passes this script into template.html for inlining.

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Project	Deployment	Actions	Updated (UTC)
api-docs-tooling	Ready	Preview	Apr 25, 2026 5:43pm

[avivkeller]

This seems highly complex

[TusharThakur04]

the {$themeScript} literal is being replaced by newly created themeScript by generate.mjs

it is done so that the script is available before rendering the html and there is no latency which could have happened if browser makes request for the themeScript.mjs if we use src inside the script tag

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.94%. Comparing base (3d00fa8) to head (ae47eff).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/generators/web/utils/processing.mjs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #766      +/-   ##
==========================================
- Coverage   78.43%   77.94%   -0.49%     
==========================================
  Files         157      159       +2     
  Lines       13962    14058      +96     
  Branches     1152     1152              
==========================================
+ Hits        10951    10958       +7     
- Misses       3006     3095      +89     
  Partials        5        5              

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[TusharThakur04]

@ovflowd
please review, i ve made the changes you suggested

[canerakdas]

LGTM, I just left one comment 🚀