feat(repo): open several repositories at once

[danielss-dev]

Summary

Batch-open multiple repositories, each as its own tab.

• Multi-select picker — pickRepoDirectory → pickRepoDirectories (multiple: true). The Open action (⌘O / topbar + / command palette) now takes several folders at once.
• Multi-folder drag-drop — dropping several folders onto the window opens all of them, instead of only payload.paths[0].
• App.openMany — opens the batch sequentially via the existing openByPath (progress popup + per-repo error handling), mirroring how restoreSession loops. Sequential on purpose: parallel opens would race the shared active-tab state and the single progress popup. A non-repo folder in the batch fails without aborting the rest; the last to open successfully ends up active.

No backend changes — the Tauri layer was already multi-repo (open_paths set, per-path watchers, path-parameterized commands).

Also includes a small UI tweak: the repo tab/rail context-menu action is shortened from "Customize icon…" to "Customize…" in both layout modes.

Test plan

• pnpm --filter ./ui exec tsc --noEmit — green.
• Manual (needs the running app): ⌘O → select 2–3 folders → each opens as a tab; drag 2+ folders onto the window → each opens as a tab; a non-repo folder in the selection surfaces an error without blocking the others.

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