Sub-issue: import and re-transcribe audio

## What
Add an import flow for existing audio files so users can transcribe old recordings.

## Why
This turns EchoNote from a live recorder into a more useful meeting archive tool. It also gives users a way to recover value from recordings made outside the app.

## How
- add a file picker / drag-and-drop import flow
- copy or reference imported audio into app storage
- reuse the transcription pipeline on imported files
- save imported sessions in the same meeting history view
- support re-transcription when model settings change

Parent: #51

Start by tracing the existing live recording flow, transcription pipeline, and meeting history entry point; the issue does not name specific files or tests. Compare those paths with the requirements for file picking, storage, imported sessions, and re-transcription. Done means imported audio can be transcribed and appears in meeting history with model-setting changes supported.

Written by the indexing model from the issue text.