feat(threads): record and display model provenance in threads

[cashcon57]

Visual

Describe Your Changes

Adds model provenance to threads: a quiet divider line showing which model
(and backend) serves the responses that follow, as discussed on Discord.

Recording. Each assistant message gets
metadata.modelProvenance = { modelId, providerId, backend? } stamped as it is
generated, riding the same metadata pipeline that already persists
finishReason / tokenSpeed / usage. The stamp is emitted at stream start
(so it survives an aborted generation) and repeated on finish. backend is the
version_backend setting of the serving provider (e.g. the llama.cpp
TurboQuant build tag), read at send time so a later backend upgrade cannot
rewrite the history of already-generated messages. No storage schema change.

Display. Divider positions are derived from the stamps at render time
(memoized walk in the thread view):

• First stamped response: Served by <model id> above the prompt that led to it.
• Any later change of model, provider, or backend build: Switched to <model id>
  above the prompt following the switch. The backend build being part of the
  identity is deliberate: the same model on a different TurboQuant build is a
  provenance change worth surfacing.
• Regenerating with a different model anchors the line to the regenerated
  response itself (the original response above it was served by the old model).
• Switching models without sending anything adds nothing; threads that predate
  this feature pick up provenance from their next response onward.

UI. Hairline rule with the model id in small muted text, bookended by the
app's asterisk mark (transparent-logo.png, faded, theme-inverted), deliberately
quiet so it does not compete with the conversation. Hovering shows a tooltip
with the model id, the standard provider display name (via getProviderTitle,
so TurboQuant and upstream llama.cpp are distinguished), and the backend build
when recorded. Strings are localized (en only; other locales fall back to
English per the i18n setup).

Tests. Unit tests for the derivation walk: first response, no-change,
model switch, provider-only change, backend-build-only change, regenerate
anchoring, pre-feature history, malformed stamps.

Fixes Issues

• Closes Record model provenance in threads (which model/backend served each response) #173

Self Checklist

• Added relevant comments, esp in complex areas (why the stamp is captured at send time, why backend is part of the identity, anchoring rules)
• Updated docs (for bug fixes / features): n/a
• Created issues for follow-up changes or refactoring needed: n/a

[cashcon57]

Apologies for the edits, I forgot to push the final edit that is represented in the image above. All good now!