feat(themes): modular plug-n-play colorscheme system

[venkatarenduchintala]

What

Refactors the theme configuration into a plug-n-play, one-file-per-theme system.

• lua/themes/<name>.lua — each colorscheme is now its own file returning a lazy.nvim spec. The 13 existing themes are migrated verbatim (including gruvbox with its original hard/high-contrast tuning).
• lua/theme.lua — rewritten as an auto-discovering loader. It scans lua/themes/*.lua, keeps the existing public API (colors, set_active_theme, get_active_theme, theme_name) so bufferline, scrollbar and plugins/init.lua keep working, persists the active choice to a machine-local state file, and adds a :Theme command with completion. Only the active theme plugin is ever installed.
• lua/plugins/configs/lualine.lua — statusline switched to theme = "auto" so the bar follows any colorscheme (the previous theme_name string errors on themes without a matching lualine theme, e.g. zenbones, tokyonight_moon).

New low-eyestrain dark themes

Added five medium-contrast, warm, desaturated themes: gruvbox_material_soft, everforest, kanagawa, tokyonight_moon, zenbones (requires lush.nvim).

Usage

• Add a theme → drop a file in lua/themes/. No central edits.
• Switch → :Theme <name> (tab-completes), then restart (lazy installs it on next launch).
• Default still set in settings.lua via set_active_theme(...); a :Theme choice overrides it.

Validation

• luac -p on all 20 changed files — clean.
• Headless loader tests: 18 themes discovered, resolve + persist + unknown-name guard all pass.
• Full devcontainer CI suite run locally (Neovim v0.12.3): green (exit 0), startup + full plugin load + all 11 treesitter parsers pass.

Note (out of scope)

Discovered that the CI reports green while swallowing headless Lua errors (nvim --headless -c qall exits 0 after an error). Pre-existing, unrelated to this change; left as-is per discussion. Candidate for a follow-up PR (switch to nvim -l, add theme launch tests, fix after/lsp/*.lua to the 0.12 return {...} form).