feat: create workspace from new branch button in Branches sub-tab

[yeikel16]

Summary

Adds a + button to the right of the branch search input in the Branches sub-tab of the "Create from…"
workspace composer. Clicking it opens a small dialog where the user can type a new branch name and
immediately create a workspace rooted at that branch — without leaving the sub-tab or switching to the
Quick composer.

Previously, users who typed a branch name that didn't exist yet had no direct path forward from the
Branches sub-tab. They had to manually switch to the Quick tab and retype the name.

Changes:

• CreateFromTab.tsx: wraps the search row in a flex container; adds the + button (visible only on the
  Branches sub-tab, disabled while a launch is in progress); adds a Dialog with a branch-name input that
  calls launchFromBranch on confirm.
• launch-work-item-direct.ts: adds an optional explicitName field to LaunchFromBranchArgs so the caller
  can pass the user-typed branch name directly instead of falling through to the creature-name generator.

Screenshots

Testing

• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm build
• No new tests added — the project has no React/jsdom test infrastructure so component-level UI tests are
  not possible without a separate setup. The core name-resolution logic (getWorkspaceSeedName with
  explicitName) is already covered by src/renderer/src/lib/new-workspace.test.ts. The launchFromBranch path
  exercised by this feature is the same one used by existing branch-row selections (no new code paths in
  the main process).

Resolve #1400