feat: follow terminal cwd and download directories

[htazq]

Summary

• add an opt-out Follow terminal directory toggle to the integrated browser
• inject a small Bash PROMPT_COMMAND hook that emits standard OSC 7 working-directory reports without parsing visible prompts
• scope the latest directory to each terminal/tab and preserve the last good browser listing when a reported path is inaccessible
• add Download directory to the integrated browser by reusing the existing recursive transfer dialog
• let normal SSH connections open one SFTP subsystem for recursive enumeration, instead of requiring a standalone SFTP tab

Why these issues are together

#69 and #70 meet at the integrated SSH file-browser boundary and share the same path/session isolation work. I did not bundle other open issues:

• Bug: terminal clears all scrollback after only 2 MiB of cumulative output #60 already has PR fix: keep PTY output flow control bounded #66 and touches pty-terminal.tsx plus its activation tests
• PR Fix four review issues from PR #59 folder-context-menu feature #65 also touches App.tsx/locales, but fixes the unrelated connection-folder menu flow
• [Bug] Edit menu commands do nothing in active terminal #56, Feature request: Highlight inactive terminal tabs when new output arrives #15, and Feature request: allow pressing R to reconnect from a disconnected terminal #12 already have separate PRs

Behavior covered

• cd with absolute and relative paths, cd .., cd ~, cd -, pushd, and popd
• spaces, Unicode, percent encoding, %, #, and ?
• active-tab/connection isolation and reconnect remounts
• global persisted follow toggle; file-browser navigation never sends cd
• inaccessible paths keep the previous path/listing and warn once until a successful load
• recursive directory creation/download, progress, per-file errors, and cancellation

Validation

• pnpm test — 39 files, 525 tests passed
• pnpm build — passed
• pnpm i18n:check — 950 keys in both locales
• ESLint on every touched frontend source file — 0 errors
• cargo test --lib --locked in Linux/Tauri container — 108 passed, 6 ignored
• cargo check --locked in Linux/Tauri container — passed
• cargo clippy --locked — completed; only existing repository warnings
• real Docker OpenSSH test — Bash OSC 7 follow cases and same-session SFTP Unicode/nested listing passed
• git diff --check — passed

Repository-wide pnpm lint remains red on the same 3 pre-existing errors in connection-dialog.tsx and settings-modal.tsx. Repository-wide cargo fmt --check also remains red on pre-existing formatting outside these hunks; both CI jobs are existing baseline conditions (Rust formatting is continue-on-error).

Known boundaries

• If sudo -i or su - starts a fresh login shell that discards PROMPT_COMMAND, following resumes only when that shell itself emits OSC 7 or after returning to the instrumented Bash shell. This change intentionally does not edit remote rc files, preserve unsafe environment variables through sudo, or rewrite user commands.
• Mixed file+directory multi-selection download is not included; the requested right-click directory download path is complete and reuses the existing progress/cancel UI.

Closes #69
Addresses #70

[GOODBOY008]

@sunxiaobin89 Can you help to review this PR？