fix(clipboard): restore empty clipboard after dictation

[erning]

Summary

When the clipboard was empty before a dictation session, restoreIfUnchanged checked backedUpItems.count == 0 and returned early without clearing the clipboard. The dictated text was permanently left in the clipboard instead of being restored to the original empty state.

Root cause: the code could not distinguish "never backed up" (should skip restore) from "backed up an empty clipboard" (should clear contents to restore).

• Add a hasBackup boolean flag set in backup and cleared after restore
• scheduleRestoreAfterDelay: and restoreIfUnchanged now use hasBackup as the guard
• restoreIfUnchanged always calls clearContents, then only calls writeObjects if the backup had items

Test plan

• xcodebuild passes
• Clear the clipboard completely, run a dictation session — after ~1.5s the clipboard returns to empty
• Copy some text, run a dictation session — clipboard restores to the original text
• Copy some text, run a dictation session, then manually paste something else within 1.5s — clipboard is not overwritten by the restore

[missuo]

LGTM. Clean fix for an edge case that would annoy users.

• hasBackup flag correctly distinguishes "never backed up" from "backed up empty clipboard"
• Guard changes from backedUpItems nil-check to hasBackup are consistent across both scheduleRestoreAfterDelay: and restoreIfUnchanged
• Properly resets hasBackup = NO after restore and on clipboard-changed-since-write bail-out
• Also properly cleans up backedUpItems = nil in the bail-out path (was missing before)