Migrate to `SecureTextField` (#1191)

* Using `SecureTextField`

Signed-off-by: Arnau Mora Gras <arnyminerz@proton.me>

* `OutlinedSecureTextField` doesn't support `readOnly`

* fixed string conversions

* - Update AddWebdavMountScreen to use enabled instead of readOnly
- Ensure onKeyboardAction checks canContinue before proceeding
- Update UrlLogin and EmailLogin to ensure onKeyboardAction checks canContinue before proceeding
- Update InputDialogs to ensure confirmEnabled is checked before proceeding

* Use get() for deriving things from a mutable state

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Signed-off-by: Arnau Mora Gras <arnyminerz@proton.me>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>