[Feature Request] "Wait for Word Completion" / Spacebar Delay for active typing

Currently, Harper scans text and underlines errors letter-by-letter in real-time. While the speed is impressive, it creates a frustrating user experience during active typing because red error lines appear in the middle of a word before it is fully typed.

For example, typing the word "the" will often trigger an error underline on the "t" and "th" before the word is finished. This is highly distracting, especially when used alongside other autocomplete extensions.

The Proposed Solution
Please add a setting toggle (e.g., "Check on Word Boundary" or "Wait for Spacebar") that forces Harper to pause its scan on the current word until the user types a space or punctuation mark.

Current behavior: Harper checks the word instantly on every keystroke.

Desired behavior: Harper checks the word only after the spacebar is pressed, indicating the word is complete.

Why this matters
Most mainstream spell-checkers (like Microsoft Edge's built-in tool or LanguageTool) default to checking on word completion. Adding this as an optional setting would make Harper much more comfortable for fast typists and prevent the screen from flashing red while simply typing normal words.

Start by tracing the real-time scan entry point and the existing settings path. Determine where the current per-keystroke check is triggered and how an optional word-boundary mode can be applied. Done means the setting is optional, scans wait until a space or punctuation mark, and existing immediate checking remains unchanged.

Written by the indexing model from the issue text.