Users with light terminal backgrounds cannot read the current color
scheme. The hardcoded dark theme colors have poor contrast on light
backgrounds.

Add a --theme flag to select between dark (default) and light themes:

  tuicr --theme dark   # default, original colors
  tuicr --theme light  # optimized for light terminal backgrounds

Introduce a Theme struct that holds all color definitions and thread it
through the rendering code. All style functions in src/ui/styles.rs now
take a &Theme parameter instead of using hardcoded constants.

The light theme uses subtle backgrounds (very light green/pink) with
dark text for good contrast on light terminal backgrounds.

Closes: agavra#7
Closes: agavra#44

Users running tuicr without knowing the available options had no way to
discover them. The --theme flag added in the previous commit is not
discoverable without documentation.

Add --help and -h flags that print a brief usage message showing all
available command-line options and a hint about the in-app help.

Document why we use a handrolled argument parser instead of clap: to
keep binary size small and build times fast.

The syntax highlighter was using hardcoded dark theme colors for diff
backgrounds and syntect theme selection. Users with light terminal
backgrounds got unreadable syntax highlighting.

Pass the SyntaxHighlighter through VcsBackend trait methods instead of
using global state. The Theme now owns a cached highlighter instance
(via OnceLock) configured with the appropriate colors and syntect theme.

For light theme, use 'base16-ocean.light' syntect theme and light
background colors for additions/deletions.