feat(themes): apply color themes in regular (non-powerline) mode (fg-only) - #607

Addresses the feature half of #604 — applying the built-in theme catalog in regular (non-powerline) mode. The U+E0B0 separator-font fallback (the bug half) is deliberately left for a separate change, so the issue stays open.

The same diff is merged and live-verified in our fork: axisrow#12.

What

Makes the built-in theme catalog usable in regular (non-powerline) mode: a new top-level theme setting cycles the chosen theme's segment palette across widgets as foreground colors.

Semantics

• theme: "dracula" (any POWERLINE_THEMES name) cycles theme[level].bg[] as widget foregrounds — bg[] is the vivid segment palette, while fg[] holds the dark text colors meant to sit on those segments. ansi16-level bg* names are converted through the existing bgToFg() helper.
• Explicit per-widget colors win over the theme; the theme beats widget default colors (Feature: use the built-in color themes in non-powerline mode (+ font-safe separator fallback) #604's proposed override semantics). Explicit colors keep their palette slot, so adding one accent doesn't shift the rhythm of the surrounding widgets.
• Separators and flex separators are not themed and consume no slot (powerline filters them out before indexing — same rule).
• Merged widgets share one slot; preserves-colors widgets keep their intrinsic ANSI output and still advance the slot — both mirror the powerline renderer.
• Unset / 'custom' / unknown name → no theming. The level follows colorLevel, using the 16/256/truecolor calibrations already shipped with each theme.
• Additive optional schema key: old configs parse unchanged, no version bump. ccstatusline set theme <name> / get work through the existing generic allowlist — zero CLI code changes.

Rework: uniform coverage across all themes

• Tests: a parametrized matrix now covers every POWERLINE_THEMES entry (custom excluded) at every color level 1/2/3 — each theme cycles its bg[] palette through the identical slot rule — plus a guard that every theme ships a non-empty palette at all levels.
• TUI parity: the theme selector now serves regular mode too (new mode prop): the main menu gains a "Theme" entry (hidden while powerline mode is enabled — powerline has its own picker), with a None entry that clears the setting. The powerline-only "copy theme to widget colors" flow remains powerline-only.
• CLI validation: validate reports unknown theme names as warnings (valid stays true; warnings array in the --json payload); set theme <name> warns on unknown names while still writing them, so a typo never invalidates a config. No schema change — unknown names keep rendering as a silent no-op.

Verification

• bun test: all theme/menu/CLI tests green (see note below). Covers cycle order, explicit-wins + slot rhythm, separators unthemed, preserves-colors, merged widgets sharing a slot, unset/'custom'/unknown names, the all-themes × all-levels matrix, the regular-mode selector writes (settings.theme set/clear), and the CLI warning paths.
• bun run lint clean; bun run build OK.
• Render check through the built binary with a theme: "dracula", colorLevel: 3 config: uncolored widgets emit exact truecolor codes (38;2;189;147;249, 38;2;248;248;242, …), separators stay unthemed; a theme: "nord" config renders the Nord palette the same way.
• CLI: validate with an unknown theme → exit 0, OK … (with warnings), machine-readable warnings array listing the known themes; set theme nord → get --json reports "theme":"nord".
• Live run as the active Claude Code status line (theme set, per-widget colors temporarily removed): all widgets picked up the cycled palette in the real render path; config restored afterwards.

Note: the repo's ink-based TUI tests have pre-existing timing flakiness on a loaded machine — the same tests (ItemsEditor, TerminalWidthMenu, ImportPreviewDialog, UsageLocaleEditor) fail intermittently on a clean checkout too. All tests touched by this PR are deterministic and green.

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