feat(themes): add gruvbox-light theme

[kanlac]

Summary

• Adds `gruvbox-light` to the built-in theme list, pairing with the existing `gruvbox-dark`
• Uses the canonical Gruvbox Light palette (`bg0` `#fbf1c7` through `bg3` `#bdae93`), with dark-variant accent colors (`#076678`, `#79740e`, `#9d0006`, etc.) for legible contrast on the light background
• Works with `autoThemeFollowsSystem` (PR feat(themes): follow macOS Appearance and auto-switch dark/light #32) by setting `"lightTheme": "gruvbox-light"` and `"darkTheme": "gruvbox-dark"`

Test plan

• Open theme picker (`t` inside sidebar), select `gruvbox-light`, verify colors render correctly
• Set `"theme": "gruvbox-light"` in `~/.config/opensessions/config.json`, restart server, verify theme loads on startup
• Verify `gruvbox-dark` is unaffected

[inspect-review]

inspect review

Triage: 2 entities analyzed | 0 critical, 0 high, 1 medium, 1 low
Verdict: standard_review

Findings (0)

---

Reviewed by inspect | Entity-level triage found 0 high-risk changes

[rs545837]

Heads up: the Rust TUI (ratatui-migration branch, PR #36) now has all 20 theme palettes ported from themes.ts. When this PR merges, we'll need a companion change to add the gruvbox-light palette to packages/sidebar-core-rs/src/renderer.rs (the palette_for_theme() match and a new const GRUVBOX_LIGHT: Palette).