fix(subscriptions): prevent newlines in topic and alias textarea inputs

[ysfscream]

Summary

• Prevent Enter key from inserting newlines in subscription topic and alias textarea fields
• Strip any existing newline characters when saving subscriptions
• Applied to both desktop and web versions

Related to #2001 and #1874

Test plan

• Open a subscription form and try pressing Enter in the topic field - should not insert newline
• Open a subscription form and try pressing Enter in the alias field - should not insert newline
• Paste text with newlines into topic/alias fields and save - newlines should be stripped

[Copilot]

Pull request overview

This PR prevents newline characters from being entered or saved in subscription topic and alias fields by adding keyboard event handlers and sanitization logic.

Changes:

• Added event handlers to prevent Enter key from inserting newlines in topic and alias textarea inputs
• Added regex-based sanitization to strip any newline characters before saving subscriptions
• Applied changes consistently to both web and desktop versions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
web/src/components/SubscriptionsList.vue	Added @keydown.enter.native.prevent and @keyup.enter.native.prevent handlers to topic and alias inputs, plus newline stripping in saveSubs()
src/components/SubscriptionsList.vue	Added @keydown.enter.native.prevent and @keyup.enter.native.prevent handlers to topic and alias inputs, plus newline stripping in saveSubs()

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