Plans -> Add Purchases: 'Number of Purchases' field needs inline range validation (Plans 5.4)

[cristim]

Symptom (Plans 5.4)

Existing Plan -> Add Purchases modal -> "Number of Purchases" field accepts values outside the documented 1-52 range. The validation message only appears when the user clicks "Add Purchases" to save. Other range-validated fields in the app surface the error inline as the user types (see PR #714 / issue #702).

Acceptance

• The Number of Purchases field shows the inline validation message as soon as a value outside 1-52 is typed (mirror the pattern from frontend/src/modules/plan-modal.ts or wherever fix(plans): live range/digit validation on plan-creation number fields #714 landed).
• The Save button stays disabled while the value is invalid.
• Regression test covers both below-range (0) and above-range (53) inputs.

Source

Plans 5.4, exploratory testing 2026-05-27. Same family as #702 (PR #714 merged).

[cristim]

Verified shipped: PR #776 (commit ba708e3, merged 2026-05-28) is in feat/multicloud-web-frontend. plans.ts Add Purchases modal now shows inline 1-52 range validation on Number of Purchases field; Save button disabled while invalid. Tests in plans-range-validation.test.ts.