adding load configurable TOML themes from DATA_DIR/themes

[aaravmaloo]

Description

This PR adds configurable themes using TOML files loaded from DATA_DIR/themes.

Summary of changes

• Added theme loading from DATA_DIR/themes/*.toml.
• Kept existing theme selection flow:
  • CHIBI_THEME override still works.
  • persisted DB theme still works when env var is not set.
• Added validation for required theme keys in TOML files.
• Added theme.ThemesPath() helper.
• Updated chibi theme output to tell users they can add their own .toml theme and where to place it.
• Added 5 example themes:
  • themes/default.toml
  • themes/nord.toml
  • themes/sunset.toml
  • themes/forest.toml
  • themes/graphite.toml
• Updated README theme docs for TOML + DATA_DIR/themes.
• Added tests for loading themes from data dir and filename-based theme naming.

Fixes #(issue) (If your PR is a 🐛 fix)

Type of change

Please delete options that are not relevant.

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update

Tested Environments

• Linux (any distro)
• Windows (10 or 11)
• MacOS ARM64
• MacOS Intel

Checklist

• I have used Gitmojis for my commit messages.
• My code follows the style guidelines of the Go language.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (leave it unchecked if your changes doesn't need documentation update)
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (Ignore this for now 🙂)
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

N/A (CLI behavior and config loading changes).

Additional context

Validation run locally:

• go test ./...

[CosmicPredator]

LGTM!!!