Reset a project thread expansion when its folder collapses

[Tryanks]

Collapsing a project resets only that project’s expanded thread list. Reopening in wide layout shows the normal six-thread limit and Show more. This also applies when the folder was collapsed in compact layout. Other projects’ expansions and parent/child folds are preserved, and direct Show less continues to work.

Closes #354.

The change stays in SessionsSidebar’s project-header transition, shared by wide and compact layouts. DESIGN documents the interaction and the existing compact full-list behavior. Existing English and Chinese labels are reused.

Evidence:

• Actual sidebar click regression failed before the fix because collapsing retained the expanded list; it passes after the fix. The test exercises Show more, both header layouts, reopening, preservation of another project and child folds, and direct Show less.
• Final native verification on macOS 26.6.2 arm64, head 0391c0b37598258a82ec29d5d09b1503cf8b8d8c: expand alpha and beta, collapse alpha, reopen alpha. In both light/dark themes, alpha shows threads 0–5 + Show more while beta retains threads 0–6 + Show less; both child folds remain closed. Retained, inspected PNGs: /tmp/tcode-354-evidence/light-reopened.png and /tmp/tcode-354-evidence/dark-reopened.png (1152×768). Compact-collapse→wide behavior is covered by the actual sidebar regression and prior native verification.
• Final local checks on 0391c0b37598258a82ec29d5d09b1503cf8b8d8c passed: cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo build --workspace --locked, and cargo test --workspace --locked (1,084 passed, 0 failed, 5 explicitly ignored across reported suites).
• Dependency hygiene passed with cargo-machete . version 0.9.2; this environment’s cargo machete forwards the subcommand as a directory, so the same installed executable was invoked directly.
• iOS, Web and Android checks passed with RUSTFLAGS='-D warnings', using the workflow’s targets and Android NDK 27.1.12297006. Native debug linking reports an existing large __eh_frame warning; Cargo reports the existing block future-incompatibility notice.
• Signed isolated final build is recorded with head/checksums in /tmp/tcode-354-evidence/build.json. The isolated app was launched with verified throwaway data/worktree environment and stopped after capture. All required GitHub CI checks passed on this exact head: run 34221950756. The standard gh run watch --exit-status completed with exit 0.

Final code, local results and retained screenshots were independently reviewed. Integration with main after #380 was checked with a clean merge tree; the sidebar implementation is unchanged from the tested head.