refactor: extract mesh state

[zatevakhin]

what changed

• extract the complete 15-field mesh UI/application slice into private MeshState
• move mesh focus/invite form types and pure transitions into src/mesh_state.rs
• retain exactly eight cross-state App coordinators for popup, diagnostics, commands, remote identity/cwd, and attach behavior
• migrate ACP, handler, and UI callers without compatibility forwards
• add focused owner and cross-boundary characterization coverage

behavior preserved

• valid node cursor and stale per-node session lists survive node replacement
• remote session cursor, offset/limit, invite validation/errors, and fallback consumption remain unchanged
• remote create keeps cwd: None; attach keeps load-then-subscribe and detached refresh ordering
• remote location/cwd, opaque attach snapshot/config, and reconnect attach-first behavior remain session/runtime-owned and unchanged

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 -- --quiet (816 passed; 3.09s harness, 3.294s wall)
• git diff --check