fix(auth): add client-side email format & required field validation

[nandhanay]

Summary

Fixes #531, #532, and #533.

Previously, auth forms and addon URL inputs accepted blank fields or invalid email formats without client-side validation, causing unnecessary backend auth calls and unresponsive submission feedback.

Changes

• Auth Site: Added validateFormInputs() in index.html to validate non-empty fields and email regex before sending requests.
• Web Settings: Disabled Addon Install button when input is blank.
• Android TV Settings: Added isConfirmEnabled property to InputModal to disable Confirm button and D-pad Enter when input is blank or not a valid http(s):// URL.

[ProdigyV21]

Thanks, this is a useful UX improvement. I tested the exact merge against current main, but one required fix remains: SettingsScreen.kt uses graphicsLayer without importing androidx.compose.ui.graphics.graphicsLayer, so both Play and Sideload Kotlin builds currently fail. I confirmed that adding that import fixes the build.

Also, the PR says it fixes #532 and #533, but these changes do not touch the Stremio migration login form. Please either add the required/email validation there as well, or update the PR scope so those issues are not marked as fixed. As a smaller improvement, the addon URL check currently accepts incomplete values such as https://; using a proper URL parser would make the validation accurate.