Add optional export filename, tests for post-implement features

Add optional filename field to export dialog with automatic .json
extension handling. Extract resolveFilename() for testability. Add
tests for includeHistory flag, bundleToJson, and filename resolution.
Add export format compatibility note to CLAUDE.md.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

specs/007-json-import-export/spec.md

@@ -73,7 +73,7 @@ A DM accidentally selects a wrong file (a non-JSON file, a corrupted export, or
 
 - **FR-001**: The system MUST provide an export action accessible from the UI that lets the user download the application state as a `.json` file or copy it to the clipboard.
 - **FR-002**: The exported data MUST contain the current encounter (combatants and turn/round state) and player character templates. Undo/redo stacks MUST be includable via a user option (default: excluded).
-- **FR-003**: The exported file MUST use a human-readable filename that includes context (e.g., date or encounter info).
+- **FR-003**: The exported file MUST use a human-readable default filename that includes the date. The user MAY optionally specify a custom filename; the `.json` extension is appended automatically if not provided.
 - **FR-004**: The system MUST provide an import action accessible from the UI that lets the user choose between uploading a `.json` file or pasting from the clipboard.
 - **FR-005**: On import, the system MUST replace the current encounter, undo/redo history, and player characters with the imported data.
 - **FR-006**: The system MUST show a confirmation dialog before importing if the current encounter is non-empty (has at least one combatant).