[AndreaCovelli]

Summary

Fixes #88. Editing any field of a host, even just its name, erased a saved password or key passphrase that couldn't be decrypted. The host then stopped being reported as unreadable, so backups exported it without its secret.

• Saving: HostProfileDao keeps the stored value when an empty one is saved over a secret that can't be opened. A typed value still replaces it, and an empty value still clears a secret that opens normally.
• Clearing: only the new Clear button erases an unreadable secret, on Save, through replaceDiscarding. Turning the passphrase switch off or picking another key no longer erases one.
• Notice: the Authentication section reports each unreadable secret whichever auth method is selected, because such a host blocks backup export even when the secret isn't in use.
• Copying: sealed values are bound to their row, so a copy can't carry the secret. Its form says the secret wasn't copied, with no Clear button, and the original host is unchanged.

UnsavedGeneratedKey is the generated-key cleanup moved out of AddServerViewModel, with no behaviour change. The new method put the class at Detekt's function limit.

Deleting an SSH key still clears the passphrase of hosts that use it through clearKeyReference, readable or not. I left that as is, since the passphrase has no key left to unlock.

Testing

• mise run check passes.
• New SealedDaoTest cases cover keeping, replacing, discarding, and clearing a readable secret. With the old overwrite temporarily restored, the keeping and replacing tests fail.
• New AddServerFormTest cases cover the notices and the clear state.
• Not run: no device was available, so the notice and the Clear button haven't been seen on screen, the view model's load and save wiring has no direct test, and the instrumented tests were only compiled. The native bridge, SSH, Mosh, and the terminal aren't touched.

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