fix(transcriber): fall back to engine default on a foreign model ID

[sumerc]

Independent of the Qwen POC (#28) — found while testing it, but it affects
Parakeet ↔ Whisper switching today.

The bug

A local provider reads its model ID from config.json and hard-errors when that
ID belongs to a different engine. That is the normal state right after any
provider switch: the persisted ID is whatever the previous provider had
selected, and it reaches the new provider via SetModel before the tray ever
offers this engine's model list.

Result: every recording fails with unknown <engine> model "<other-engine-id>"
until the user happens to open the model menu and pick something.

The fix

Fall back to the provider's own default instead of erroring
(transcriber/local.go).

Safe because the default is by construction the right engine's file. What must
never happen is loading another engine's weights — that is a multi-minute hang
rather than a clean error, per the note at local.go:102 — and that is still
rejected.

Tests pass; Parakeet and Whisper both verified transcribing after the change.

🤖 Generated with Claude Code

[upsun-dispatch-dev]

📋 PR Summary

This PR changes local (on-device) transcription providers so that when config.json holds a model ID belonging to a different engine — the normal state right after a provider switch — load() falls back to the engine's own default instead of hard-erroring every recording. It touches only transcriber/local.go (the fallback branch plus a new defaultID field) and adds a CHANGELOG entry.

Changes

Layer / File(s)	Summary
local provider fallback
transcriber/local.go	Adds a defaultID field and a fallback branch in load(): when the desired model ID is unknown or belongs to another engine, rewrite modelID to this engine's default rather than returning an unknown <engine> model error. Loading another engine's weights is still rejected.
changelog
CHANGELOG.md	Documents the behavior change that local providers no longer fail every recording on a foreign persisted model ID.