feat(web-ui): Agent App session picker and Studio prompt editor

[GCWing]

Summary

• New session dialog: Loads enabled user Agent Apps from the catalog API when the dialog opens, extends the agent dropdown with those apps, and restores a previously stored agent choice when it matches either a built-in mode or a loaded app id.
• Agent App Studio panel: Replaces read-only prompt block with MarkdownEditor (read-only when manifest is readonly), adds explicit Save/Cancel when the draft is dirty, persists via agentAppAPI.updateAgentApp, and adjusts SCSS so the prompt tab fills remaining height.
• Lint: Moves MetaRow, StatTile, and SectionHeader above the main panel component to satisfy no-use-before-define.

Verification

• pnpm run type-check:web
• pnpm run lint:web