feat(ui): split environment picker into project/collection dropdowns [INS-2793]

[ZxBing0066]

INS-2793

Summary

• Splits the single "Manage Environments" popover into two independent dropdowns — project-level and collection-level — each with a scope tooltip and an inheritance-order hint (folder overrides collection, which overrides project).
• Replaces the hidden "+" menu in the collection environment editor and the project-level full-page environment editor with explicit, always-visible "Add Sub Environment" / "Add Private Sub Environment" buttons.
• Adds an "Add Project Environment" action to the project dropdown (does not auto-select the new environment).
• Adds a keyboard shortcut for the project dropdown (Cmd/Ctrl+Alt+E), alongside the existing collection-environment shortcut (Cmd/Ctrl+Shift+E).
• Fixes smoke tests broken by removing the old single-picker trigger, and adds new coverage for the split dropdowns and create flows.

Test plan

• npm run lint — clean (only 2 pre-existing unrelated warnings in generated vendor files)
• npm run type-check — clean (packages/insomnia and insomnia-data)
• Verified visually against the running app: independent dropdown open/close, tooltip copy, long-name truncation, project-environment creation flow, explicit sub-environment buttons
• Ran updated/new Playwright smoke tests against the real app (environment-editor-interactions, global-environments) — passing