Paginate the Archived Threads settings page and add Delete all

[Tryanks]

Why

Settings → Archived Threads rendered every archived thread as a row with two buttons and click listeners. With thousands of archived threads the page froze on open.

What

• Render 50 rows per page, grouped by project as before, with a "x–y of N" caption and Previous/Next buttons (disabled at each end). A project spanning a page boundary repeats its caption on the next page.
• Add a Delete all danger button that always confirms (independent of the skip-confirm preference) and then dispatches one DeleteSession per archived thread.
• Locale strings in en and zh-CN; DESIGN.md updated.

Skipped a virtual list: the settings page scrolls as a whole, and 50 rows per page is cheap.

Evidence

• cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings: clean
• cargo test --workspace --locked --no-fail-fast: all passing, including the new page_bounds unit test and the locale parity test