feat(projects): support adding multiple projects at once in the directory picker

Add a multi-select mode to the "Add project directory" dialog: each
directory row gets a select toggle (checkbox icon, Space toggles the
highlighted row), and the primary action becomes "Add selected" and
registers every selected directory in one store update. Selections apply
to the currently browsed directory and reset on navigation, dialog open,
and clone-mode entry. Clone mode keeps its single-target flow.

Add addProjects() to useProjectsStore: validates, normalizes, and dedups
paths (already-added or duplicated), creates entries in a single state
update and single persist, activates the first newly added project, and
discovers icons for each entry. Mirrors addProject semantics for the
single entry.

Refs OPE-142

CHANGELOG.md

@@ -10,6 +10,7 @@ All notable changes to this project will be documented in this file.
 - Usage: GitHub Copilot now shows a single AI Credits window, matching Copilot's token-based quota, in place of the old Chat Requests and Completions windows (thanks to @jakoss).
 - Settings: fixed the Cloudflare Tunnel download link shown when cloudflared is not installed (thanks to @AyoubAchour).
 - Git: picking a remote branch such as `origin/main` in the branch selector now switches you to that branch instead of leaving the repository on a detached `HEAD` with no branch name.
+- Projects: the "Add project directory" picker now accepts multiple selections at once — each row has a checkbox (or press Space on the highlighted row) and the primary action adds every selected directory in a single store update, with per-entry validation, deduplication, and one persistence write (thanks to @herjarsa).
 - Desktop: "Restart to Update" no longer looks dead when the update cannot be installed — the update window now shows the reason, including when the running copy was not installed from an official signed release, and the button stays available to retry.
 
 ## [1.21.0] - 2026-08-26