feat(widget): add TotalCost widget for cumulative cross-session cost tracking

[badoriie]

Summary

Adds a new total-cost widget that tracks cumulative Claude Code spend across all sessions — complementing the existing session-cost widget which only shows the current session.

How it works:

• On each status line render, the current session cost is persisted to <config-dir>/costs/<session-id> (alongside settings.json)
• The widget sums all cost files and displays the running total
• Session IDs are sanitized before use as filenames
• Uses the ccstatusline config directory so it respects --config path overrides

Usage:

{ "id": "1", "type": "total-cost", "color": "green" }

Renders as Total: $12.34 (or $12.34 in raw/minimalist mode).

Test plan

• 10 new unit tests covering: preview, persistence, multi-session summing, update-on-render, zero-cost skipping, raw mode, session ID sanitization, and corrupted file resilience
• All new tests pass (vitest run src/widgets/__tests__/TotalCost.test.ts — 10/10)
• Pre-existing test failures (10 files, 81 tests) confirmed present on upstream main before this change — not introduced here