Add --account flag to search command

[davidggphy]

Adds --account flag to msgvault search to scope results to a single account when multiple accounts are synced into the same archive.

Problem

With multiple accounts synced, search returns results from all of them interleaved, with no way to filter at query time:

$ msgvault search "budget proposal"
# results from alice@personal.com and alice@work.com mixed together

Solution

$ msgvault search "budget proposal" --account alice@work.com
# only messages belonging to alice@work.com

Implementation

Resolves the account identifier to a source_id via GetSourceByIdentifier and sets q.AccountID directly on the parsed search.Query. buildSearchQueryParts already handles AccountID, so no changes to internal packages or the Engine interface are needed. The lookup runs before ensureFTSIndex so an invalid account fails immediately.

Closes wesm#164.

Testing

go test -tags fts5 ./... passes. No new lint issues introduced (golangci-lint run ./... reports only pre-existing issues in other files).

Tested manually against a two-account archive:

# Filters to one account — returns only that account's messages
$ msgvault search "budget proposal" --account alice@work.com

# Same query on the other account — returns no results (correct)
$ msgvault search "budget proposal" --account alice@personal.com
No messages found.

# Unknown account — fails fast with a clear error
$ msgvault search "budget proposal" --account unknown@example.com
Error: account "unknown@example.com" not found