chore: simplify frontend dependencies and tooling

[Azganoth]

Summary

• replace Immer-backed Zustand updates with native immutable partial updates
• replace the local task-to-promise wrapper with Promise.try
• remove deprecated @types/babel__core and rely on Babel's bundled declarations
• remove the obsolete @/components/dialogs import restriction from Oxlint
• reduce redundant application code and direct dependencies without changing intended behavior

Related Issue

Not applicable

Verification

• pnpm exec vitest run src/features/preferences/stores/settings.test.ts src/features/preferences/stores/recentItems.test.ts verifies preference-store behavior without Immer middleware.
• pnpm exec vitest run src/lib/async.test.ts src/lib/cancellation.test.ts verifies synchronous errors and promise-returning tasks remain correctly handled by the async and cancellation utilities.
• pnpm check:frontend verifies formatting, frontend types and linting, the production build, and the coverage suite.
• Not verified: manual application behavior; no user-facing behavior is intended to change.

Notes

• Leafdown no longer directly depends on or uses Immer. The lockfile still resolves Immer as an optional peer of Zustand.
• No changelog entry was added because this is an internal maintenance cleanup.