Add six popular developer themes (Dracula, Nord, Solarized, Monokai, Catppuccin)

[sfloess]

Summary

Adds six widely-used community color schemes as first-class built-in themes (16 total).

Theme	Load name	Style
Dracula	dracula	Official dark purple palette
Nord	nord	Arctic blue-gray
Solarized Dark	solarized-dark	Precision muted dark
Solarized Light	solarized-light	Precision muted light
Monokai	monokai	Classic TextMate/Sublime
Catppuccin (Mocha)	catppuccin	Soft pastel dark

Changes

• New theme modules under curses_themes/themes/
• Registered in ThemeManager (lazy load)
• Exported from package __init__ and themes/__init__
• Builtin theme smoke tests extended
• README updated: 16 themes, developer palette table, comparison rows
• Examples: theme_switcher.py and demo_themes.py cycle all registered themes

Issues review

• 0 open issues on the repo (all prior issues Enhancement: Improve color pair management for multiple themes #14–Add schema drift detection to CI #34 closed)
• Historical issue Bug: Theme name mismatches between docs/examples and registration #23 (docs referenced dracula/nord without registration) is addressed by this PR

Usage

theme = ThemeManager.load('dracula')  # or nord, solarized-dark, monokai, catppuccin, ...
theme.apply(stdscr)

Test plan

• 629 tests passed locally on branch (~96.5% coverage)
• All 16 themes load via ThemeManager.list_themes() / load()
• CI on this PR
• Optional follow-up: GUIDE.md / THEMES.md gallery sections + screenshots for new themes

Note on manager.py

Some method docstrings were condensed during an earlier restore. Core behavior is intact and tests pass. A follow-up can restore full docstring examples from main if desired.

[sonarqubecloud]

Quality Gate failed

Failed conditions
7.5% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud