feat(fragments): import SillyTavern standalone lorebooks

[imb101]

Summary

• Adds a dedicated Lorebook Import dialog for uploading SillyTavern standalone world info / lorebook JSON files
• Handles both array-format entries and SillyTavern's object-with-numeric-keys format ({ "0": {...}, "1": {...} })
• Per-entry selection with type-override dropdown (knowledge / guideline / character / prose)
• Preserves lorebook metadata: sticky, placement (system/user), keys as tags, insertion order, priority
• BookOpen toolbar button appears in the Knowledge section sidebar
• Drag-dropping a lorebook JSON onto the editor auto-routes to the dialog
• Zero new TS errors introduced

Test plan

• Drop a SillyTavern lorebook .json onto the story editor — should open LorebookImportDialog
• Open via Knowledge section toolbar (BookOpen icon)
• Verify entries import as knowledge/guideline fragments with correct tags and sticky flags
• Confirm constant entries land as sticky, before_char entries get system placement
• Try a lorebook with object-keyed entries (exported directly from SillyTavern)
• Confirm character card JSON and Errata bundle JSON still route to their own dialogs

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