feat: add vesper theme

[nexxeln]

summary

• add Vesper as a built-in theme
• document the new theme option

testing

• bun run typecheck
• bun run format:check
• bun test src/opentui/HunkDiffView.test.tsx src/ui/lib/ui-lib.test.ts src/ui/diff/pierre.test.ts

[greptile-apps]

Greptile Summary

This PR adds Vesper as a fifth built-in diff theme, including its own custom Pierre syntax-highlighting definition and a refactor that threads the full AppTheme object through the highlighting pipeline instead of just the appearance string.

• New Vesper theme (src/ui/themes.ts, src/ui/diff/pierre.ts): defines color tokens, distinct added/removed background pairs, and registers a custom TextMate-scoped syntax theme (hunk-vesper) at module load time via registerCustomTheme.
• API refactor (useHighlightedDiff.ts, DiffPane.tsx, PierreDiffView.tsx, HunkDiffView.tsx): the highlight cache key, prefetch function, and React hook all now accept AppTheme instead of \"light\" | \"dark\", allowing per-theme syntax palettes without changing the call sites' external contracts.
• Documentation and tests updated across README.md, docs/opentui-component.md, CHANGELOG.md, and the relevant test files to reflect the new theme name and ordering.

Confidence Score: 5/5

Safe to merge — the change is additive, limited to theme data and the highlighting pipeline, and the refactored API passes stable object references throughout.

The Vesper theme is additive and isolated. The AppTheme-propagation refactor is mechanical and the stable-reference guarantee from the THEMES array ensures no spurious effect re-runs. Tests cover token colors, background distinction, and reserved-color remap isolation for the new theme.

No files require special attention.

Important Files Changed

Filename	Overview
src/ui/themes.ts	Adds Vesper theme with distinct added/removed background colors; addresses previously flagged identical-background issue.
src/ui/diff/pierre.ts	Registers a custom Pierre syntax theme for Vesper at module load time; adds pierreSyntaxThemeName dispatch and refactors loadHighlightedDiff/pierreRenderOptions to accept full AppTheme instead of appearance string.
src/ui/diff/useHighlightedDiff.ts	Refactors cache key and all public APIs to accept AppTheme instead of appearance string; cache key now incorporates theme.id for per-theme correctness.
src/ui/components/panes/DiffPane.tsx	Updates prefetchHighlightedDiff call and its effect dependency from theme.appearance to theme; no logic changes beyond the API signature update.
src/opentui/HunkDiffView.tsx	Propagates the full resolvedTheme object to useHighlightedDiff instead of just appearance; no other changes.
src/opentui/themes.ts	Adds vesper to HUNK_DIFF_THEME_NAMES constant and the derived HunkDiffThemeName union type.
src/ui/diff/pierre.test.ts	Adds a Vesper-specific syntax token assertion and extends the reserved-color remap test to include vesper.
src/ui/lib/ui-lib.test.ts	Updates theme menu order assertion; adds addedBg/removedBg distinction assertions for the Vesper theme.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Theme prop (string)"] --> B["resolveTheme()"]
    B --> C{id in THEMES?}
    C -- Yes --> D["AppTheme (stable ref from THEMES)"]
    C -- No --> E["graphite fallback"]
    D --> F["pierreSyntaxThemeName(theme)"]
    E --> F
    F --> G{theme.id === 'vesper'?}
    G -- Yes --> H["hunk-vesper (custom Pierre theme)"]
    G -- No --> I["pierre-dark / pierre-light (built-in Pierre theme)"]
    H --> J["renderDiffWithHighlighter()"]
    I --> J
    J --> K["HighlightedDiffCode (cached by theme.id + file fingerprint)"]

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_{Reviews (2): Last reviewed commit: "fix: use vesper syntax highlighting" | Re-trigger Greptile}