fix: recognize Claude Sonnet 5 as a 1M-context model - #50

Summary

contextWindowFor() only recognized the 1M context window via a "4.6+"
version pattern (e.g. sonnet-4-6), so claude-sonnet-5 — whose model ID
has no -4- segment — fell through to the legacy 200K default. This adds a
check for bare major-version-5+ Sonnet models, so the dashboard and
status-bar context bar correctly show a 1M window for Sonnet 5. Confirmed
against the official docs:
"Claude Sonnet 5 supports the 1M token context window by default (1M tokens
is both the default and the maximum; there is no smaller context variant)."

Linked issues

None.

Type of change

• Bug fix (non-breaking) → fix (patch)
• New feature (non-breaking) → feature (minor)
• Breaking change (existing behaviour differs afterwards) → breaking (major)
• Documentation / CI / chore → docs / ci / chore (patch)

How was this tested?

• Added src/test/dataLoader.test.ts covering getCurrentContextInfo for
  claude-sonnet-5; confirmed it fails (200000) without the fix and passes
  (1000000) with it.
• npm run compile clean; npm test — all 25 tests pass.
• Manually checked the new regex against existing 4.x model IDs
  (claude-opus-4-8, claude-opus-4-1-20250805, claude-sonnet-4-5-20250929,
  claude-haiku-4-5-20251001, etc.) to confirm no regression to legacy
  classification.

Checklist

• npm run compile is clean
• User-facing strings go through I18n with all six languages filled
• CHANGELOG.md updated (user-visible changes)
• README.md updated if behaviour/settings changed — checked, no
  per-model context-window docs exist there to update
• New settings default to existing behaviour (opt-in) — not applicable,
  no new settings
• No new runtime dependencies