Add path autocomplete + folder picker to add-project flow

[Ed-Barnes937]

What

The add-project / scan-directory path input was a blind free-text field. This makes it a live path autocomplete with a native folder picker, seeded with ~/ as the base directory.

• Type to filter — directories matching what you've typed appear in a dropdown, updating as you go (debounced).
• Tab completes to the longest common prefix.
• ↑/↓ highlight a match; Enter on a highlight drills into that directory, Enter on free text commits the project.
• Browse… opens the OS folder picker as a fallback.

How

Backend (src-tauri/)

• projects.rs — new stateless complete_path(partial) -> Vec<String> command, with directory-listing logic ported from claude-commander's (private) tui/path_completer.rs. Unified tilde handling into one expand_tilde/unexpand_tilde helper (removed the old shellexpand_tilde) and applied it to add_project. Added 5 unit tests.
• main.rs — registered complete_path; added tauri_plugin_dialog::init().
• Cargo.toml / capabilities/default.json — tauri-plugin-dialog + dialog:allow-open; tempfile dev-dep.

Frontend (src/)

• main.ts renderTopInput — dropdown, debounced complete_path, keyboard handling, and the Browse button.
• style.css — dropdown + button styling (existing theme tokens).
• help.ts + README.md — documented the new keys.

Tests

• TauriSimulator — complete_path/add_project/scan_directory/dialog fakes driven by a seeded dirs list (no real filesystem).
• SidebarPageObject — path-input POM helpers.
• New addProject.iwft.ts — 5 scenarios (filter, Tab, drill-in, commit, Browse).

Verification

typecheck ✓ · cargo fmt/clippy clean ✓ · 5 backend tests ✓ · 49 Vitest ✓ · 68 iwft ✓ (incl. 5 new)

Notes

• complete_path is synchronous — one read_dir at a 100ms debounce is negligible on the main thread.
• Completions preserve the ~ form when typed (matches the TUI).
• Not manually eyeballed in the running app yet; iwft can't judge visuals.

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