feat: add min-length filter and history deduplication

[dupenodi]

Closes #4

Summary

Addresses noise in clipboard history caused by modal editors (Vim, Emacs) that yank individual characters or repeated fragments on incremental deletions. Two new opt-in settings are added under Preferences → History:

• Min-length filter — entries shorter than the configured threshold (default: 1 character, range 1–20) are silently dropped before being written to history.
• Deduplicate history — when enabled, copying a text entry that already exists in history promotes the existing item to the top instead of inserting a duplicate. (Applies to inline text < 50 KB; large file-backed entries are already deduplicated consecutively via hash in ClipboardWatcher.)

Both settings persist via UserDefaults through SettingsManager.

Changes

File	What changed
Services/SettingsManager.swift	Add minTextLength and deduplicateHistory properties + persistence
Services/ClipboardWatcher.swift	Drop entries shorter than minTextLength before adding to store
Services/ClipboardStore.swift	In performAdd, promote duplicate inline text to top when dedup is on
Views/SettingsView.swift	New "History" section with a Stepper and a Toggle; frame height bumped to 400

Test plan

• Set min-length to 3 in Preferences; copy a single character — it should not appear in history
• Copy a word that already exists in history with dedup enabled — existing entry should move to top, no duplicate added
• Disable dedup; copy the same word twice — both entries should appear
• Verify settings survive an app restart
• Verify existing history and bookmarks are unaffected

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