Add global exercise notes

[samoca95]

Problem: Currently, notes can only be added to specific routines or within a logged workout. There's no central place to store persistent information about an exercise that applies regardless of the routine or workout session. Users need to rely on memory or external apps for things like setup details or technique links.

Proposed Solution: Implement a dedicated, persistent "Notes" field associated with each individual exercise.

Key Features:

• Exercise-Specific: The note is tied to the exercise itself (e.g., "Barbell Squat"), not a specific instance in a workout log.
• Persistent: The note remains saved and accessible indefinitely until edited or deleted by the user.
• Independent: Not tied to any specific routine or workout date.
• Accessible: Easily viewable and editable from the main information page for that exercise (e.g., where the description and muscles worked are shown). Perhaps also viewable when adding the exercise to a workout.

Use Cases:

• Storing links to preferred technique/tutorial videos.
• Documenting specific machine settings or setup steps (e.g., "Seat position 3, cable height 10").
• Adding personal reminders or form cues (e.g., "Focus on driving through heels," "Keep core braced").
• Noting specific mobility drills or warm-ups related to the exercise.