build: add automatic schema URL replacement in TOML files

[pamburus]

Changes

• Implement build script to replace HTTP(S) schema URLs with relative paths
• Add SHA256 hash validation to ensure local schemas match remote versions
• Replace schema URLs in config and theme TOML files with relative paths
• Add taplo and tombi validation to CI/CD workflows
• Add tombi.toml configuration for TOML linting
• Reformat JSON schema files for consistency
• Add setup script for development tools
• Enhance justfile with TOML validation targets

The build script now automatically:

• Scans all *.toml files in the assets directory
• Detects schema directives using HTTP(S) URLs
• Validates local schemas against remote versions via SHA256
• Replaces URLs with relative paths to local schema files

This ensures offline-first operation while maintaining schema validation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.58%. Comparing base (3ef77f2) to head (4eeaef9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #471   +/-   ##
=======================================
  Coverage   76.58%   76.58%           
=======================================
  Files          29       29           
  Lines        4011     4011           
=======================================
  Hits         3072     3072           
  Misses        939      939           

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