Add preferred_language pref and route mood shifts to steer_session

[randileeharper]

Summary

Fixes #91.

Two related fixes for session-steering behavior and music language preference.

1. Music language preference (config + playlist selection)

Adds a preferred_language config setting (list[str] | None) that biases playlist selection toward the user's preferred language(s) without polluting the Apple Music search term.

• preferred_language accepts a comma-separated string ("english, spanish") or a list; empty entries are dropped.
• Surfaced in the playlist-selection Context only — the step where candidate playlist names/descriptions are visible. The session-planner search term stays clean of language qualifiers, which previously corrupted retrieval (e.g. "upbeat english-language maintanin contemporary pop for morning").
• playlist_selection.txt prefers playlists matching preferred_languages, with an explicit per-request language override (e.g. "k-pop", "spanish pop") taking precedence over the default.

2. Steer on mood/energy shift (resolver prompt)

Tightened resolver.txt so that a mood/energy/direction shift on an active session routes to steer_session with search_update {"mode": "replace"} instead of play_session, which was causing the same queue to regenerate on repeated steering attempts (e.g. "switch to something upbeat" was starting a new session rather than steering).

Test commands run

.venv/bin/python -m compileall vesper tests
.venv/bin/python -m pytest -q

All 250 tests pass.

To activate

Add to your config file:

{ "preferred_language": ["english"] }

or export VESPER_PREFERRED_LANGUAGE="english", then restart the service.