Fix - vibium storage restore fails to restore localStorage/sessionStorage

[Jhoan0714]

Summary

This PR fixes #217 — browserStorageState double-encoded the exported storage state, so vibium storage restore silently failed to restore localStorage/sessionStorage even though it reported success.

Root Cause

browserStorageState in clicker/internal/agent/handlers.go ran a script that already calls JSON.stringify(...) in the browser, and then embedded that resulting string directly into the exported state map before a second json.MarshalIndent pass — so the "storage" field ended up as an escaped JSON string instead of a nested JSON object. browserRestoreStorage interpolates that field directly into var state = %s; when restoring, so the escaped string became a JS string literal instead of an object, and state.localStorage/state.sessionStorage were undefined — nothing was restored, with no error reported.

Why this matters

• vibium storage / vibium storage restore are the only CLI/MCP commands for saving and reusing browser session state (e.g. login sessions) across runs — this bug made that workflow completely non-functional
• The failure is silent: the CLI prints "Storage state restored" with no indication anything went wrong
• No impact on Python/JS/Java clients — context.storage()/context.setStorage() use a separate, unaffected wire-API code path (vibium:context.storage/setStorage → handleContextStorage in clicker/internal/api/handlers_storage.go)

Validation

Executed:

vibium go "https://sahitest.com/demo/training/books.htm"
vibium eval "localStorage.setItem('user', 'alice')"
vibium eval "sessionStorage.setItem('session_id', 'abc123')"
vibium storage -o state.json
cat state.json
vibium eval "localStorage.clear(); sessionStorage.clear();"
vibium storage restore state.json
vibium eval "localStorage.getItem('user')"
vibium eval "sessionStorage.getItem('session_id')"

Before fix — state.json had "storage" as an escaped string, and restore silently failed:

{
  "cookies": [],
  "storage": "{\"origin\":\"https://sahitest.com\",\"localStorage\":{\"user\":\"alice\"},\"sessionStorage\":{\"session_id\":\"abc123\"}}"
}

localStorage.getItem('user')          = null
sessionStorage.getItem('session_id')  = null

After fix — state.json has "storage" as a real nested object, and restore works:

{
  "cookies": [],
  "storage": {
    "localStorage": {
      "user": "alice"
    },
    "origin": "https://sahitest.com",
    "sessionStorage": {
      "session_id": "abc123"
    }
  }
}

localStorage.getItem('user')          = alice
sessionStorage.getItem('session_id')  = abc123

Test added: