feat: add json schema for config files

[troycdc]

Summary

• Add JSON Schema definitions for both config files (config.toml and tui.toml) under schemas/configs/
• Add github module to cdcx-core with URL builders (html, api, raw, release_download) derived from CARGO_PKG_REPOSITORY, replacing hardcoded GitHub URLs in update.rs
• Generated config files now include a #:schema directive pointing to the raw GitHub URL, enabling editor validation and autocomplete
• Reorganize schemas/ directory: API overlays moved to schemas/apis/, config schemas live in schemas/configs/

Schema coverage

File	Schema
~/.config/cdcx/config.toml	schemas/configs/config.json — profiles with api_key, api_secret, environment
~/.config/cdcx/tui.toml	schemas/configs/tui.json — theme, tick rate, ticker speed, watchlist, custom themes with hex color validation

Test plan

• cargo check passes
• cargo test -p cdcx-core -- github — 5 new tests for URL builders
• Run cdcx setup and verify generated config.toml starts with #:schema directive
• Run TUI setup wizard and verify generated tui.toml starts with #:schema directive
• Open generated TOML files in an editor with TOML schema support and confirm validation works

[llcro]

nice idea - get it merged