fix(stt): make local microphone transcription work without torch (#801)

faster-whisper runs on CTranslate2, not torch, but _get_whisper()
imported torch (only to check cuda availability) inside the same try as
the faster-whisper import. on a torch-less machine that raised
ImportError and reported the misleading 'faster-whisper not installed'
even when it was installed, so local mic transcription silently failed.

probe torch separately and optionally: present -> cuda, absent -> cpu.
also declare faster-whisper in requirements-optional.txt (torch stays an
optional extra for gpu).

requirements-optional.txt

@@ -4,6 +4,14 @@
 # Note: chromadb-client + fastembed moved to requirements.txt — RAG, semantic
 # memory, and tool selection are core paths, so they ship by default now.
 
+# Local speech-to-text (microphone -> text) via faster-whisper, for the
+# "local" STT provider. Runs on CPU out of the box (CTranslate2 backend, no
+# torch needed). Install if you want to dictate/transcribe with the mic
+# without sending audio to an external endpoint.
+# Optional extra: install `torch` too if you have a CUDA GPU and want
+# GPU-accelerated transcription — it's auto-detected, CPU is used otherwise.
+faster-whisper
+
 # DuckDuckGo as a search provider option.
 # Install if you want DDG in the search-provider dropdown.
 # Alternatives: SearXNG, Brave, Tavily, Serper, Google PSE.