fix: validate shortcut filename when URL is pasted

[AlexAndBear]

Description

This bugfix makes Create Shortcut use the shared filename validator and fixes the missing inline error state after URL paste.

• Reuses useIsResourceNameValid for shortcut filename validation.
• Prevents invalid shortcut names from being submitted when URL paste auto-fills the filename.
• Keeps the filename input mounted while hidden so the inline error message appears immediately when the auto-filled name is invalid.
• Uses the shared validation result for both inline error text and confirm-button disabled state.

Related Issue

• Fixes Creating a shortcut lets you submit a invalid name #3328

How Has This Been Tested?

• test environment: local dev environment on macOS
• test case 1: paste a URL into Webpage or file without using the suggestion popup and verify invalid auto-filled filename shows an inline error and keeps Create disabled
• test case 2: provide a valid URL and valid shortcut name and verify Create is enabled
• test case 3: pnpm test:unit --run packages/web-pkg/tests/unit/components/CreateShortcutModal.spec.ts
• test case 4: pnpm check:types

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)