SplitDialogs: support Subtitle Edit Batch convert

[niksedk]

Adds Batch convert support (SubtitleEdit/subtitleedit#16036) to Split dialogs.

What it does

• batch (once per file, no UI): splits every dialog line in the file using the saved options (Only split where the line before the dash ends a sentence, Gap between lines (ms)). ASSA/SSA stay in their native format, like a normal run. A file with no dialog lines returns ok without a subtitle. Settings are not returned.
• configure (Batch convert's Configure...): the window opens with only the two options, a short "Batch convert settings" panel, and an OK button (always enabled). The preview list, column header and Select all/none are hidden. OK returns the settings + settingsVersion with no subtitle; Cancel returns cancelled.
• plugin.json has "batch": true. SE 5.3 ignores it and only sends run. run works exactly as before.

Decisions

• Batch applies what the window preselects: every row is checked by default, so batch splits every line the dialog rule accepts with the saved options. There are no per-item questions, so no extra batch-only setting was needed.
• This plugin has no view model (the logic is in the window code-behind). To share the logic with the headless path without a refactor, settings load/save moved from MainWindow to a small SplitSettings record (same JSON keys and version 1, so saved settings carry over). The batch/configure responses live in a new BatchMode class.

Tests

• New se5/SplitDialogs.Tests project (modelled on FixEncodingErrors.Tests) with BatchModeTests, 6 tests: default split, saved settings (rule off + gap), no dialogs = ok without subtitle, ASSA style/actor kept, configure response, settings round-trip/defaults. All pass.
• split-dialogs.yml now has a test job (build waits for it) and the Tests path in its filters, like fix-encoding-errors.yml.
• Ran the built dll end-to-end with a batch request.json: exit 0, the dialog split as in the README example (01,000-01,738 / 01,762-04,000).
• Not checked by hand: the configure window in a real SE.

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