feat: Dictionary — custom replacements & spoken snippets

[karansinghgit]

What

A Dictionary feature (inspired by Wispr Flow), reworked for a fully-local app. Map a spoken phrase to any output text:

• Say "my email" → inserts your address (text expansion / snippets)
• Fix a name/term the model keeps mishearing (spelling correction)

Rules run offline as a post-processing pass on the final transcript.

How it works

• DictionaryService — Codable entries persisted as JSON in UserDefaults (mirrors HistoryService). Thread-safe static apply(to:). One-time migration imports the old freeform customReplacementRules string into structured entries, so existing rules are preserved.
• Applied in TranscriptionManager.transcribe so it covers both engines (Whisper and Parakeet). The old customReplacementRules only ran on the Whisper path — Parakeet silently skipped it. Removed that duplicated code from WhisperService; filler-word removal stays behind the Auto Edit toggle.
• New Dictionary sidebar screen: add/edit/delete, per-rule enable + whole-word toggles. Settings "Transcript Cleanup" now points here.

Notes

• Matching is always case-insensitive (a spoken trigger's capitalization is the recognizer's choice, not the user's).
• CI on this branch will be red until ci: build on macos-26 so the macOS 26 SDK (.glassEffect) is available #104 (macos-26 runner) merges — the failure is the pre-existing .glassEffect SDK issue, unrelated to this change.

Test plan

• Dictionary tab → Add Rule ("my email" → an address); dictate → address is inserted.
• Existing customReplacementRules migrate into entries on first launch.
• Leaving the replacement empty deletes the phrase.