refactor: extract sessions state

[zatevakhin]

what changed

• add one private SessionsState owner with exactly 23 browser, discovery, viewport, identity/mode, new-session, activity, and remembered remote-location fields
• replace the 23 flat App fields with one pub(crate) sessions: SessionsState, reducing direct App fields from 89 to 67
• move session item/completion types, constants, matching, pagination, child merge, collapse, cursor/filter, mode, completion, activity, and remote-location behavior into the owner
• retain command construction, ACP lifecycle coordination, profile binding, filesystem/path normalization, connectivity, routing, rendering, mesh attach, and reconnect behavior at existing boundaries
• keep launch_cwd and delegate_popup_visible_rows flat; preserve the focused reset_active_session_view lifecycle boundary

scope and metrics

• exact base: 3a7a599ff0fb04def35f2ba1591d3623dc5e9ba0
• source commit: dcc88cbc2be93550c394f8e8bd6dab2ec910c2be
• exact 13-file allowlist: src/session_state.rs, src/lib.rs, src/app.rs, src/session.rs, src/acp_state.rs, src/handlers.rs, src/mesh.rs, src/runtime/mod.rs, src/runtime/event_loop.rs, src/ui/start.rs, src/ui/popups.rs, src/ui/chat.rs, src/ui/mod.rs
• diff: 13 files, 2,228 insertions, 1,709 deletions
• resulting source: 51 Rust files, 44,505 physical LOC, 67 direct App fields, five App impl files, 23 SessionsState fields, 848 test declarations

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 (848 passed, 0 failed/ignored)
• focused owner tests (14 passed) and session-focused tests (202 passed)
• git diff --check

boundary proof

• exactly one private mod session_state;, one App::sessions, and 23 owner fields
• zero old flat declarations/direct accesses and zero old crate::app::{StartPageItem, PopupItem, PathCompletionState} paths
• no forbidden owner imports beyond the explicitly allowed FileIndexEntryLite dependency
• launch_cwd and delegate_popup_visible_rows are each declared once on App and absent from the owner
• exactly four existing phase 11 compatibility forward tags remain
• no source outside the approved 13-file allowlist changed

residuals

• no manual TUI smoke claim
• phase 4, broad roadmap acceptance, connection/chat/composer/delegates/render extraction, and phase 11 cleanup remain pending
• integration is not claimed until a separately approved merge and equivalence verification