fix(files_sharing): apply link share password on first save

[pringelmann]

• Resolves: [Bug]: Adding a password to a share is not effective unless saved twice #57011

Summary

Setting a password on a public link share doesn't always apply on the first save. The input shows a value but the create-share request drops the password; you have to toggle "Set password" off and on, or save twice, for it to stick.

The isPasswordProtected setter awaits GeneratePassword then calls $set(share, 'newPassword', ...). Because newPassword isn't on the share at observation time, this goes through Vue 2's property-addition path, which only notifies renders subscribed to the parent's __ob__.dep. Whether the field's render is in that subscriber set depends on prop-access ordering at first read. Customer/enterprise builds land on the broken side; some dev setups don't, which is why this has been hard to reproduce.

Pre-declaring newPassword on _share (with a class getter/setter) gets Vue's observer to wire it up alongside the rest of the share state. $set then writes through an existing reactive property and notifications go through its own dep.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)