feat: Auto-save transcript and summary as Markdown to a custom directory

[ruzin]

Description

Allow users to configure a custom directory where StenoAI automatically saves a Markdown (.md) file of each meeting's transcript and summary after processing completes.

Context

Currently, StenoAI saves meeting data as:

• Raw transcript: .txt files in the transcripts directory
• Summaries: .json files in the output directory (structured with overview, key points, action items, etc.)
• Storage path is configurable via get/set_storage_path in src/config.py

Users want to integrate meeting notes into their existing knowledge management workflows (local folders, Dropbox, iCloud Drive, etc.) without manually copying content. A rendered Markdown file is more useful than raw JSON for human consumption.

Requirements

• Add config options: get/set_export_directory and get/set_auto_export_enabled in src/config.py
• After summarization completes (in src/summarizer.py or the pipeline in simple_recorder.py), automatically write a .md file to the configured directory
• Markdown format should include:
  • Meeting title and date as a header
  • Summary/overview section
  • Key points / decisions
  • Action items / next steps
  • Participants (if identified)
  • Full transcript (collapsible or at the end)
• Add UI in the Electron app settings to pick the export directory and toggle auto-export
• Support manual "Export as Markdown" from the meeting detail view
• Use a sensible filename pattern, e.g. YYYY-MM-DD-meeting-title.md

Notes

• Scribe mode templates (src/templates.py) have their own section structure -- the Markdown export should adapt to whichever template was used
• Related: feat: Export meetings to multiple formats #21 (export to multiple formats) -- this issue focuses specifically on the auto-save Markdown workflow rather than one-off export to various formats