fix(web): plumb has_image/has_file into search index

[intendednull]

Summary

• Indexer at crates/web/src/app.rs:382 hard-coded has_image: false / has_file: false / letter_id: None, so the documented has:image / has:file / in:#letter operators silently returned zero hits.
• New pure helper willow_client::derive_has_image_file(body) derives the booleans from the body string: inline [file:name:b64] payloads (the format produced by ClientHandle::share_file_inline) split on filename extension, and image URLs in plain text fire has:image to match the renderer's auto-embed behaviour.
• Wired the helper into the indexer Effect in app.rs. letter_id stays None for now — there is no active-letter signal in web state because letters-dms.md hasn't landed. Tracked in Search index: thread active-letter id into IndexableMessage.letter_id #448.

Tradeoffs

• Helper lives in willow-client (not willow-web) because the inline-file body format is owned by ClientHandle::share_file_inline and a future native consumer (agent / CLI) needs the same booleans. Runner-up: parse-from-Content::File directly — rejected because DisplayMessage is built from ChatMessage which only carries body: String; the structured Content enum is collapsed at projection time, so body is the only signal available without a wider refactor.
• Image URL detection mirrors the renderer's extension-list rule rather than full URL parsing — keeps the indexer cheap and matches what users visually see embedded.

Test plan

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check -p willow-web --target wasm32-unknown-unknown
• cargo test -p willow-client --lib search:: (10 new tests in derive::tests + 1 regression test in execute_tests::derived_attachment_flags_drive_has_image_and_has_file_operators)
• cargo test -p willow-web --lib

Refs #355
Follow-up: #448

https://claude.ai/code/session_01HnwK1dP6rnrsbLxtg92zfF

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Generated by Claude Code

[intendednull]

Fix conflicts please

[intendednull]

Superseded by #442. main now have IndexableMessage::from_display_message with same has_image/has_file derive logic. Closing — fix already shipped.

Test coverage on main covers same surface (search/tests.rs from_display_message_tests mod). Letter_id plumbing follow-up still tracked at #448.

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