Add experimental Claude model selection

[Manho]

Summary

• add a global experimental Claude model selector with Claude default, Opus 4.8, Sonnet 5, and Haiku 4.5
• persist and synchronize the selection across Claude panels without changing requests in the default mode
• constrain request rewriting to embedded Claude /completion requests and allowlisted model IDs
• fall back to Claude default after schema incompatibility or a 4xx response without automatically resending messages
• update localized UI copy, responsive dark/light styling, tests, and privacy documentation

Why

Claude limits its official model selector when embedded. This provides an explicit opt-in compatibility mode while keeping Claude default as the safe behavior and making account-dependent availability clear to users.

Validation

• npx vitest run --exclude='tests/e2e/**' — 271 tests passed
• npx playwright test tests/e2e/claude-model-selection.e2e.test.js --reporter=line — passed
• verified 1x1 and 1x10 layouts in light and dark themes

Notes

• This change does not update the extension version or include release artifacts.
• Model requests remain experimental and may be rejected by Claude based on the user's plan or organization policy.