fix(desktop): persist agent avatar edits

[andresuribe87]

Problem

You can't update an existing agent's profile image. Creating an agent sets the image fine, but editing it does nothing — the avatar picker in the edit dialog is even hard-disabled with a comment noting the backend can't persist it.

Root cause

avatarUrl was missing from the entire update path — this was an unimplemented feature presenting as a bug:

• The avatar picker in AgentInstanceEditDialog was disabled (comment: "UpdateManagedAgentInput has no avatarUrl field, so edits can't be persisted yet").
• UpdateManagedAgentInput (TS IPC type) had no avatarUrl field — though CreateManagedAgentInput and UpdatePersonaInput both do.
• The Rust UpdateManagedAgentRequest struct had no avatar_url field.
• The update_managed_agent handler never wrote record.avatar_url, and only re-published the kind:0 profile when the name changed (re-using the old image even then).

So the create path carried the avatar; the edit path silently dropped it — hence the bug is specific to updating an agent.

Fix

Wire avatarUrl end to end:

• Add avatarUrl?: string | null to UpdateManagedAgentInput (tri-state).
• Resolve + send it from the edit dialog — base64 data URIs upload to a hosted URL, emoji SVGs pass through, mirroring the create path via resolveManagedAgentAvatarUrl — and enable the picker. On upload failure the helper falls back to the existing avatar, so a transient failure never clobbers the current image.
• Add avatar_url: Option<Option<String>> to UpdateManagedAgentRequest (via double_option, matching provider).
• Assign it to the record and re-publish the agent's kind:0 profile when the avatar changes (previously rename-only).

Tri-state semantics: absent = don't touch · null = clear to the harness default avatar · url = set.

Testing

• New Rust tests: avatar_url deserialization tri-state (absent / null / value), mirroring the existing provider tests — pass.
• pnpm typecheck (desktop) — clean.
• biome check on changed files — clean.
• Desktop JS unit tests (incl. managedAgentAvatar) — 11/11 pass.
• cargo fmt --check and cargo clippy — clean for the changed files. (Two pre-existing clippy dead_code warnings in archive/store.rs + an unused import in agent_settings.rs are unrelated to this change.)

Manual verification steps

1. Edit an existing managed agent, pick/upload a new avatar, Save.
2. Confirm the avatar updates in the agent list and the change persists across restart.
3. Clear the avatar → reverts to the harness default.

🤖 Generated with Claude Code