Add client-side length caps + truncation for Shop name/description

[dadachi]

Summary

Ports nativeapptemplate/NativeAppTemplate-iOS#60. Mirror the ItemTag pattern from #49 for Shop. Server has no caps on Shop name/description; this is a client-only UX guard.

• Constants: `maximumShopNameLength = 100`, `maximumShopDescriptionLength = 1_000`. New strings: `shopNameIsInvalid`, `shopDescriptionIsInvalid`, plus `shopNameHelp(maximumLength:)` / `shopDescriptionHelp(maximumLength:)` parametric helpers.
• `ShopCreateViewModel` + `ShopBasicSettingsViewModel`: split `hasInvalidData` into `hasInvalidDataName` + `hasInvalidDataDescription`, expose `maximumNameLength` / `maximumDescriptionLength`, add `validateNameLength()` / `validateDescriptionLength()`.
• `ShopCreateView` + `ShopBasicSettingsView`: wire `.onChange` truncation on Name and Description; switch to two-line footer (always-visible help + conditional red "is invalid" text), matching `ItemTagCreateView`.
• Tests: added `maximumNameLength`, `maximumDescriptionLength`, parametric `nameValidation` / `descriptionValidation`, and the two truncation tests on both view-model test suites; replaced the old simple `hasInvalidData(name:)` parametric test in `ShopCreateViewModelTest`.

Test plan

• `xcodebuild build-for-testing` — `** TEST BUILD SUCCEEDED **`
• `make lint` — `0 violations` (SwiftLint + SwiftFormat)
• `xcodebuild test` passes (please verify in Xcode with Cmd+U). New parametric tests should cover boundary cases at 100 / 101 / 1000 / 1001 chars.
• Manual:
  • Open Add Shop, type 105 chars in Name → field truncates to 100, no red error. Save stays enabled.
  • Type 1500 chars in Description → truncates to 1000.
  • Clear Name → red "Shop name is invalid." appears, Save disabled.
  • Same on Shop Settings → Basic Settings.

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