feat(translate): add text-mode Kagi Translate support

[Microck]

Summary

• add kagi translate for Kagi Translate text mode
• bootstrap translate_session from the existing KAGI_SESSION_TOKEN
• return one JSON envelope with detection, translation, alternatives, alignments, suggestions, and word insights
• add translate docs clarifying the default --from auto --to en behavior
• add a recorded terminal demo to the README and docs
• harden translate validation and output metadata before review
• refactor translate bootstrap to use the same pure-Rust HTTP stack as the rest of the CLI

Implementation

• normalize Session Link values at runtime for env and config auth
• bootstrap translate auth by fetching https://translate.kagi.com/ with reqwest and extracting translate_session from Set-Cookie
• call direct HTTP endpoints for detect, translate, and the text-mode auxiliary sections
• stabilize the translate envelope by always backfilling translation.source_language and translation.target_language
• reject invalid kagi translate --to auto requests with a config error before hitting the backend
• downgrade optional-section failures into warnings while keeping the core translation result
• add translate-focused unit tests plus ignored live tests gated by KAGI_SESSION_TOKEN
• document the command, auth matrix, output contract, error cases, and demo workflow
• add scripts/demo-translate.sh and ship generated GIF assets for docs previews
• make the existing demo scripts directly executable by the documented asciinema rec -c ./scripts/demo-*.sh flow
• remove the temporary python3 + curl_cffi runtime dependency and revert the extra Tokio process feature

Verification

• cargo test
• cargo test live_translate -- --ignored --nocapture
• cargo build
• ./target/debug/kagi translate "Bonjour tout le monde"
• ./target/debug/kagi translate "Bonjour tout le monde" --no-alternatives --no-word-insights --no-suggestions --no-alignments
• ./target/debug/kagi translate "Bonjour tout le monde" --to auto
• ./scripts/demo-translate.sh
• asciinema rec --overwrite -c ./scripts/demo-translate.sh -q -i 0.2 --cols 92 --rows 22 /tmp/kagi-demos/translate.cast
• agg --theme asciinema --font-size 14 --idle-time-limit 2 --last-frame-duration 4 /tmp/kagi-demos/translate.cast docs/demo-assets/translate.gif

Notes

• scope is text mode only
• translate now uses reqwest end-to-end rather than a Python bootstrap helper