Add light theme support via toml config

[dimroc]

Before	After

Why

oit's footer status bar uses dark RGB backgrounds (Color::Rgb(40,40,40) in status_bar.rs, Color::Rgb(30,30,30) in command_input.rs), and the process color palette in process_colors.rs relies on light/bright variants (LightGreen, LightYellow, LightCyan, …). On a light-background terminal these wash out: the footer hint line is illegible and process names blend into the background.

The existing comment in process_colors.rs already calls out the dark-terminal assumption — this PR makes it a real choice instead of a constraint.

What

Introduces a single, simple toml option:

theme = "light"   # or "dark" (default)

Unknown values silently fall back to dark, so existing configs are untouched.

Light palette

Takes color inspiration from gruvbox light. Avoids bright variants, picks darker hues with good contrast on cream/white backgrounds:
Blue, Magenta, Rgb(0,120,0), Rgb(120,80,0), Rgb(0,100,140), Rgb(140,0,140), Rgb(0,100,100), Rgb(160,0,80), Rgb(80,80,140), Rgb(100,60,100)

Implementation

• New src/ui/theme.rs — a Theme struct holds named tokens (footer bg/fg, muted, accent, selection bg/fg, process palette, fallback) plus Theme::dark(), Theme::light(), and Theme::from_config(Option<&str>).
• Config — adds theme: Option<String> (skip-serialize-if-none, no migration needed).
• App — gains theme: Theme, populated from config in main.rs before any color setup.
• ProcessColors::new — now takes palette: &[Color] and fallback: Color from the active theme instead of a hardcoded module-level palette. Precomputes ANSI start-codes per process and supports 24-bit Color::Rgb and Color::Indexed so the light palette's RGB colors render inline in log lines (the dark palette still maps to standard 16-color ANSI exactly as before).
• Widget updates: status_bar.rs, command_input.rs, and the three selection blocks in log_viewer.rs read tokens off app.theme instead of hardcoded RGB values. Other Color::DarkGray borders/separators were left alone — they're readable on both backgrounds; we can revisit if needed.

Backwards compatibility

• Default theme is dark — visually identical to today.
• The theme field is optional; missing/None → dark. Dark tokens preserve the previous exact RGB values.
• No snapshot tests were affected (verified — no *.snap.new produced).

Tests

• cargo test — 487 unit + 562 lib + all integration suites green.
• New tests:
  • src/config.rs — test_theme_loads_from_config, test_theme_defaults_to_none_when_missing
  • src/ui/theme.rs — from_config parsing (case-insensitive, unknown → dark, None → dark) and light-palette ordering
  • src/ui/process_colors.rs — test_light_palette_assignment, test_rgb_color_emits_truecolor_ansi; existing dark-palette tests refactored to pass the palette explicitly

Manual verification

1. cargo build --release
2. Add theme = "light" to a project's .overitall.toml, run oit in a light terminal — the [TAIL] footer line and the "Press : for commands…" hint are readable.
3. Remove the line, restart — dark behavior unchanged.

Docs

• README.md Configuration Options table updated.
• man/oit.1 updated.
• oit --init now generates a .toml with a # theme = "dark" hint