fix(budget-tracker): use Decimal for precision, add edit & delete-by-…

[Rachi-collab]

What

• Replaced all float usage with decimal.Decimal for monetary values to eliminate floating-point precision errors (e.g. 0.1 + 0.2 no longer gives 0.30000000000000004)
• Added edit_transaction() — lets users update category, description, amount, or type of any existing entry by ID without touching other records
• Added delete_transaction() — lets users remove a single transaction by ID after confirmation
• Added an id column to the CSV schema; existing files without it are auto-migrated on startup

Why

Fixes all three issues reported in #1718:

• Financial calculations were imprecise due to float arithmetic
• There was no way to correct a wrong entry without wiping everything
• There was no way to remove one transaction selectively

How

• Decimal with ROUND_HALF_UP to 2 decimal places is used everywhere amounts are read, stored, or calculated
• save_all_transactions() rewrites the full CSV atomically, used by both edit and delete
• next_id() generates auto-incrementing integer IDs; initialize_file() detects and backfills IDs in old CSV files for backward compatibility
• Menu options renumbered: Edit → 6, Delete single → 7, Clear all → 8, Exit → 9

Testing

• Added income and expense entries and verified totals are exact with no floating-point drift
• Edited individual fields (category, description, amount, type) one at a time; pressing Enter skips and keeps the original value
• Deleted a single transaction by ID and confirmed all other entries remain intact
• Ran python -m py_compile to verify no syntax errors
• Tested auto-migration: ran the updated script against an old CSV (no id column) and confirmed IDs were backfilled correctly

Closes #1718

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