Autocorrect-on-space with backspace undo and rejection learning

[Pivii]

Context

Once the trie spell-correction engine (#7) lands, wire it into the typing flow the way iOS does (v1.3.0-beta + #67/#75/#107 correction-UX overhaul):

• Autocorrect on space: committing a space replaces the misspelled word with the top correction (iOS-native feel)
• Backspace undo: backspace right after an autocorrect restores the original typed word
• Rejection learning: an undone correction adds the word to the user dictionary — never corrected again (iOS UserDictionary)
• Suggestion bar integration: left slot = verbatim typed word (quoted), center = top correction (bold), right = alternative
• Numeric/mixed tokens (test123, h2o) must not be autocorrected (iOS fix in v1.4.0-beta.1)
• Respect the autocorrect settings toggle (see LanguageProfile issue)

Dependencies

Blocked by #7 (trie engine). Interacts with the existing PersonalDictionary (learns words typed ≥2 times) — merge both learning paths into one user dictionary.

Acceptance criteria

• Misspelled word + space → corrected, with visual cue in suggestion bar
• Backspace immediately after → original restored and learned
• Learned words are never corrected again (persisted)
• Numbers/mixed tokens untouched