Browser-local package cache indexing

[jonsherrard]

Summary

• move package browsing/search to a client-only IndexedDB index
• add onboarding for choosing browser-readable package caches
• support pnpm stores, Bun install caches, and Yarn zip caches with centralized dedupe
• document cache relocation and multi-cache indexing behavior

Verification

• pnpm run typecheck
• pnpm run build
• static server smoke: / and /onboarding return the SPA

Note: the in-app browser bridge was not exposed in the current Codex tool set, so the final UI smoke used the built static server responses rather than interactive browser automation.