Add multiple save file support with save slot management

[Copilot]

Enables users to maintain multiple independent save files and manage them via an interactive menu at startup.

Changes

SaveFileManager (src/saveFileManager.py)

• Manages save slots in data/slot_N/ directories
• Lists saves with metadata (day, money, fish count, timestamp)
• Handles slot creation, deletion, and gap-filling (reuses deleted slot numbers)

FishE initialization (src/fishE.py)

• Displays save selection menu before game start
• Routes all save/load operations through selected slot path
• Supports creating new saves, loading existing, and deleting unwanted slots

Example interaction:

 FISHE - SAVE FILE MANAGER
---------------------------------------------------------------------------

 Available Save Files:

 [1] Save Slot 1
     Day: 10
     Money: $1500
     Fish: 50
     Last Modified: 2026-02-05 10:02:32

 [2] Save Slot 2
     Day: 2
     Money: $100
     Fish: 5
     Last Modified: 2026-02-05 10:02:32

 [N] Create New Save (Slot 3)
 [D] Delete a Save File
 [Q] Quit

Each slot contains isolated player.json, stats.json, and timeService.json files. Saves are fully independent with no cross-contamination.

Tests

• Added 15 tests covering slot management, metadata reading, and edge cases (corrupted files, missing data)
• Updated existing FishE test to mock save selection menu

Original prompt

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This section details on the original issue you should resolve

<issue_title>Multiple save files</issue_title>
<issue_description>As a user, I would appreciate the ability to have multiple save files and be able to detect/manage these saves when the application starts.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Multiple save files #51

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