perf: precompute session-scoped style set to eliminate per-frame allocations

[deveshctl]

Summary

• All theme-derived lipgloss.Style values are now built once at startup into a themeStyles struct (tui/styles.go) and reused for every frame. The theme is immutable for the session, so the ~300–400 per-frame style allocations that were rebuilding identical structs on every View() call are eliminated.
• The pre-built style set is stored on the model as m.styles and threaded through the internal render structs (treePaneInput, splitTreeInput, viewerParams) so the hot per-row renderers read a field rather than calling lipgloss.NewStyle().
• Dynamic styles — those whose color depends on per-row state (diff type, delta sign, cursor selection, search match position) — remain as inline allocations since their inputs vary per call.
• All test call sites updated to pass themeStyles{} (zero value is correct for structural tests that check rendered content, not color).

Changes

• tui/styles.go — new themeStyles struct (26 fields) and newThemeStyles(Theme) constructor
• tui/model.go — styles themeStyles field on model; static calls replaced in renderHeader, renderStatusBar, renderViewerStatusBar, viewLoading, viewError, separator line
• tui/filetree.go — treePaneInput/splitTreeInput carry styles; static calls replaced in renderTreeHeader, renderFilterBar, renderSplitDivider, formatFileNodeLine, renderNameWithHighlight
• tui/layers.go — static calls replaced in renderCommandBar, highlightInstruction, formatLayerLine, renderSizeColumn
• tui/fileview.go — viewerParams carries styles; static calls replaced in renderViewerLine, renderViewerSearchBar, overlayCursor, gutter, and binary/empty/truncation notices

Test plan

• go build ./... and go vet ./... clean
• CI passes (go test ./...)
• On a real image: navigate the file tree, toggle sort, open the viewer, activate search — visual output identical to before; held-key scroll and search responsiveness improved on large layers