refactor: extract composer state

[zatevakhin]

what changed

• replace 11 flat composer fields on App with private ComposerState
• move UTF-8 input editing, visual movement/layout, mention/file-index state, and slash completion into the owner
• keep command construction, prompt effects, editor status, and cross-owner App::take_input coordination at their existing boundaries
• preserve session reset survivors while clearing file-index and mention session data
• delete src/input.rs after migrating its composer-local behavior

validation

• cargo fmt --all -- --check
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets --all-features
• cargo test --all-targets --all-features (860 passed, 0 failed, 0 ignored)
• git diff --check

scope

• base: refactor/components@8fb8257d25e4b120236315f5b5277f3a8f9b6c61
• phase 4 ComposerState slice only
• no phase 5 events/effects, other phase 4 owner, protocol, transport, command, config, or persistence changes
• source review intentionally left for an independent reviewer