fix(hindsight): show embedded config in desktop

[aikeepsitreal]

Summary

Fixes #84572 by making Desktop represent existing Hindsight local_embedded configuration honestly without bypassing the setup flow that installs its runtime dependencies.

• preserves an existing local_embedded mode as a disabled, read-only select option instead of displaying the Cloud default
• exposes the embedded LLM provider, base URL, model, and secret fields in Full config
• maps the embedded LLM secret to HINDSIGHT_LLM_API_KEY
• keeps selecting local_embedded setup-owned, while allowing edits to an already configured embedded installation
• adds backend and Desktop behavior coverage

Root cause

The generic declared-config serializer replaced stored select values outside a surface's write allow-list with the field default. Hindsight intentionally omits local_embedded from Desktop's writable Mode options because hermes memory setup owns dependency installation, but that caused a valid setup-created value to render as Cloud. The declared schema also omitted the embedded LLM fields and secret mapping.

User impact

Users with Hindsight Local Embedded configured through setup now see their actual mode and LLM settings in Desktop and can edit those settings safely. Desktop still cannot create a partially installed embedded configuration.

Validation

• 152 passed, 1 skipped across the current Hindsight schema and full web-server test files
• 10 passed in the Desktop provider-config panel test
• Ruff passed
• Prettier 3.9.5 passed
• git diff --check passed

[alt-glitch]

This was generated by AI during triage.

Related: #84586 and #58924 address the same Hindsight Desktop configuration family through broader declared-schema changes. This PR preserves existing setup-created local_embedded values as read-only rather than making Desktop select that mode; reviewers should choose the desired configuration model.