Add-in: sidecar-backed thread fetcher (ERMAIN-503)

[bdart]

What changes

Wires the desktop sidecar into the Outlook add‑in's thread loading (ERMAIN‑503): on the on‑prem / Exchange‑SE path, conversation loads go through the sidecar instead of makeEwsRequestAsync (whose size caps degrade SE threads), with transparent fallback to the existing EWS path on any failure — never worse than today.

• Decorator (fetchOutlookMessageSidecar.ts): wraps the EWS fetcher, overrides only fetchConversationMessages, gated on sidecar availability + a resolvable anchor/mailbox; maps the result to GraphConversationMessage[] so ParsedThread, dedup, disclosure markers and .eml synthesis are unchanged.
• Injection (useOutlookMessageFetcher.ts): applies the decorator on the on‑prem branch when the sidecar client is present; the cloud/Graph path is untouched.
• Endpoint (env.ts): bridges the configured VITE_DESKTOP_SIDECAR_URL to the provider's runtime lookup — without it the provider stays inert.
• Re‑exports the handful of protocol types the add‑in needs from @erato/frontend/library.

Verification

Add‑in typecheck + lint + prettier clean; tests green (decorator cases: mapped result, and EWS fallback on unsupported / no‑mailbox‑match / empty). Verified end‑to‑end against a live Exchange‑SE mailbox.

Stacked on #887. Retarget to main once the stack lands.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YNRaKxV2kk7SfPJN6sccME