Fix spell upcasting bugs and add scaling editor

[Fronix]

Summary

Closes #321

• Mana slider bugs: Safe property access prevents the 100-mana slider (Pyroclasm) and 0-mana minimum (Fiery Embrace). NPCs without mana no longer crash the dialog.
• Tier fallback fix: Null/undefined highestUnlockedSpellTier now defaults to 9 instead of 0.
• Separate upcast & higher level effects: New upcastEffect field separates upcast descriptions (extra mana spent) from higherLevelEffect (character-level scaling). Both display independently in the description editor, summary, chat card, and tooltip when populated.
• World migration: Moves higherLevelEffect to upcastEffect on world load, but only for spells with upcast scaling configured. Spells without upcast scaling keep their higher level effect untouched.
• Spell scaling editor: Visual CRUD editor for scaling mode, deltas, and choice groups. Target effect uses a dropdown from activation effects with a text fallback for new spells.
• JSON editor fallback: Toggle between visual editor and raw JSON editing of the full spell config.
• i18n: All editor strings use translation keys.

Test plan

• Open Pyroclasm (tier 4) on a character with < 100 mana — slider should cap at available mana
• Open Fiery Embrace (tier 5) — slider minimum should be 5, never 0
• Open upcast dialog as an NPC — should not crash
• Tier 1+ spell with upcast scaling — "Upcast Effect" tab appears in description editor
• Cantrip — only "Base Effect" and "Higher Level Effect" tabs, no "Upcast Effect"
• Tier 1+ spell — "Upcast" section shows in summary, chat card, and tooltip
• Add content to both fields on a tier 1+ spell — both sections display independently
• World migration only migrates spells with upcast/upcastChoice scaling mode (check console logs)
• Spells with scaling.mode = 'none' keep higherLevelEffect untouched after migration
• Scaling editor: add/edit/remove deltas and choices
• Target effect dropdown populates from activation effects; text fallback with tooltip on new spells
• JSON editor toggle works and shows full system data
• Run existing tests: npx vitest run

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