Add grammar check via LanguageTool

[ivandrofly]

Summary

A new Tools → Grammar check (LanguageTool)… window that checks the subtitle against a LanguageTool server - the public API or a self-hosted one (e.g. the docker image) - and applies the fixes the user ticks. It complements AI review: rule based, so it points at an exact span and offers concrete replacements instead of rewriting a line, it is much faster, and with a self-hosted server the text never leaves the machine.

• src/libuilogic/Grammar/ — LanguageToolClient (/v2/check, /v2/languages, picky level, disabled rules, optional premium credentials), LanguageToolAnnotatedText (builds the annotated document and maps the answer back), LanguageToolFix (applies several fixes to one line). Kept out of the UI project so seconv can use it later.
• src/ui/Features/Tools/GrammarCheck/ — the window and its view models. Server box with a test-connection button, language list served by the server (defaulting to the auto-detected subtitle language), picky toggle, category filter chips, a checkbox grid with before/after diffs, a replacement picker for the selected row, and a small settings dialog for user name / API key / disabled rules / lines per request.
• Menu entry (both the normal and the native macOS menu), DI registration, Se.Settings.Tools.GrammarCheck, localizable strings, and a docs page.

Formatting tags, ASSA override blocks, music symbols and the line break inside a subtitle are sent as markup, so LanguageTool neither checks nor counts them as words, while the offsets it returns still point into the original line - a fix lands exactly where it belongs and the tags stay untouched. An issue spanning a tag or a line break is dropped rather than applied. Lines whose sentence continues into the next subtitle are joined with interpretAs, so a sentence is checked the way it reads on screen and agreement errors spread over two lines are found; fixes are still applied per line, so timing and reading speed are unaffected.

Nothing is applied automatically: spelling/grammar/punctuation/casing fixes start ticked, style suggestions start unticked, and rules that offer no replacement cannot be ticked at all.

Test plan

• Tools → Grammar check (LanguageTool)… opens; the language drop-down fills and the status line reads "Connected - N languages available"
• Point Server at an unreachable address and press the refresh button - the error is shown in the status line, the window stays usable
• Check an English subtitle containing e.g. <i>He go to school</i> / I has a apple - issues appear per line with category tags and before/after diffs
• Select a row with several replacements - the drop-down under the grid appears, picking another option updates the After column
• Filter with the category chips; counts match the number of rows in each category
• Untick some rows, press Apply N fixes - only the ticked fixes are applied, several fixes on one line all land correctly, and Ctrl+Z reverts the whole run in one step
• A line wrapped in <i>…</i> or starting with {\an8} keeps its tags after applying a fix inside it
• A sentence split over two subtitles gets its agreement error reported, and applying the fix does not move words between the lines
• Press Stop mid-check - what was found so far stays in the grid
• Settings… - user name/API key/disabled rules/lines per request survive a restart; a rule id in "disabled rules" makes that rule stop firing

UI

🤖 Generated with Claude Code

[ivandrofly]

I also have https://languagetool.ivandrofly.org that any one can use or you can run your own https://github.com/Erikvl87/docker-languagetool

[niksedk]

Hi @ivandrofly, thanks a lot for the work on this - the tag/line-break handling and the cross-subtitle sentence joining are really nicely thought out, and the UI looks good.

I'm going to close this one for now though. The main sticking point is the dependency on a LanguageTool server: either the text goes to the public API, or the user has to run a Java/Docker setup themselves, and neither fits well with where SE 5 is heading (everything running locally, no external services by default).

Longer term I'm hoping we can do an AI-assisted port of the LanguageTool rule engine to C#, so the grammar check can run fully in-process with no server at all. If that materializes, a lot of what you built here - the annotated-text mapping, the fix application, the review grid - could come straight back, so please don't throw the branch away 🙂

Thanks again!