Add a "Create Custom Exercise" flow, accessible from the exercise library page (e.g., a button near the search bar).

Form fields:

• name (required, text input)
• exercise_type (required, select: strength / cardio / mobility / other)
• measurement_type (required, select: reps_weight / time / distance / reps_only)
• primary_muscle (optional, select dropdown — options limited to the existing ENUM: chest, back, shoulders, biceps, triceps, forearms, abs, quads, hamstrings, glutes, calves, full_body)
• equipment (optional, free text input)

On submit:

• Insert a new row into exercises with the form values, is_custom = true, and created_by set to the logged-in user's user_id.
• Validate that name isn't empty and exercise_type/measurement_type are valid enum values before submitting.
• After a successful save, close the form and return the user to the exercise library with the new exercise visible in the list, tagged with the existing "Custom" badge, without requiring a manual page refresh.

Edge cases to handle:

• Prevent duplicate submissions if the user double-taps "Save."
• Show a clear inline error if the insert fails (e.g., a network issue), without losing the user's entered form data.
• Since primary_muscle is an ENUM column, make sure the dropdown can only submit valid values — no free text that could violate the constraint.

Keep styling consistent with the rest of the exercise library screens.