fix(embedding): treat the built-in local backend as available

[mrsibe]

What does this PR do?

Makes the Knowledge Base panel treat the built-in local embedding backend as configured, instead of showing "Please configure a default embedding model in settings first" and disabling Add Source.

Why?

SourcePanel computed hasEmbeddingModel from connections.embedding:

const connections = await window.api.connections.getAll()
setHasEmbeddingModel(Boolean(connections.embedding))

Selecting the built-in local backend clears that connection (that is the point of the local mode), so the check was always false for the default configuration. The result:

• the "configure a default embedding model" empty state was shown,
• Add Source was disabled,
• file / URL / paste / note import were blocked by alerts.

Since #54, embedding is infrastructure: EmbeddingService.resolveBackend() falls back to the built-in local model when no remote connection exists, and ensureReady() downloads the weights on first RAG use. Availability therefore has to come from the real backend, not from the presence of a connection.

Related issue

No open issue; reported from manual testing. Related to #54.

What changed?

• SourcePanel now reads window.api.embedding.getStatus() and sets hasEmbeddingModel when activeBackend === 'local' or a remote connection is configured.
• Refreshes on connections.onChanged (local ↔ custom switch) and on embedding.onDownloadProgress (install state changes).

How was this tested?

Verified with Electron CDP against npm run dev, with no embedding connection configured (i.e. built-in local backend active):

• Before: Knowledge Base / Please configure a default embedding model in settings first, and Add Source was disabled=true.
• After: the empty state is gone and Add Source is disabled=false, so documents can be added and the local model downloads on first use.

Also checked:

• npm run typecheck:web passes.
• npm test passes (49/49).
• npx prettier --check src/renderer/src/components/notebook/SourcePanel.tsx passes.

Screenshots / recordings

Not captured (text/state change only).

Checklist

• I have reviewed my own changes.
• npm run typecheck passes.
• npm run build passes.
• I have tested the affected user workflow.
• I have not included unrelated changes.
• I have updated documentation when necessary.

Desktop / build changes

• Not applicable