Summary

Fixes #1750.

This adds a full-terminal live thinking stream so long reasoning output can be monitored as it arrives without being squeezed into the compact transcript pane.

What changed

• Added a /thinking slash command, with /think as a short alias, that opens a thinking-only live overlay.
• Extended the existing LiveTranscriptOverlay with a ThinkingOnly mode that:
  • refreshes from the active transcript snapshot every frame, so streaming reasoning continues updating while the overlay is open;
  • filters the overlay to HistoryCell::Thinking entries only;
  • renders thinking cells with full-detail transcript rendering instead of compact summaries;
  • keeps the existing sticky-tail controls so the user can either follow the newest reasoning or scroll away and resume tailing with End/G.
• Updated Ctrl+O Activity Detail behavior for in-flight thinking: when the selected/current activity is still streaming, it opens the live thinking stream; finalized thinking still opens the existing reasoning timeline pager.
• Added localized command descriptions for the shipped locales.

Verification

• cargo fmt --all -- --check
• cargo test -p deepseek-tui live_transcript --all-features
• cargo test -p deepseek-tui execute_thinking_opens_live_reasoning_stream --all-features
• cargo test -p deepseek-tui activity_detail_opens_live_thinking_stream_for_streaming_thinking --all-features
• cargo test -p deepseek-tui command_registry_has_unique_names_and_aliases --all-features
• cargo test -p deepseek-tui localization --all-features
• cargo clippy -p deepseek-tui --all-targets --all-features

Note: clippy still reports existing warnings in schema_migration.rs and existing needless_return warnings in ui.rs; no new blocking warnings were introduced by this change.