[EPAC-1978]: switch sheets to adaptive form presentation

[riddim-developer-bot]

Reviewer-Boundary: review-only

Scope

• Updates regularSizeClassFormSheet so regular horizontal size classes use form-style sheet sizing with medium/large detents and a visible drag indicator.
• Applies the adaptive sheet modifier to non-onboarding app sheets: postal-code setup, settings, what's new, explainer/data-source details, party-line info, and member compare picker.
• Leaves onboarding on the existing sheet path because the shared helper documents onboarding as an exception.

Bugfix SPEC

• Spec: N/A — feature implementation for Linear issue EPAC-1978.
• Trace ID: N/A.

Testing notes

• Automated tests run: cd ios && make test TEST_ONLY=AdaptiveLayoutTests — passed, 2 tests.
• Build: cd ios && make build — passed.
• Static checks: python3 scripts/localization/check_localizations.py --github-warnings — passed. swiftlint --strict <9 touched Swift files> — passed with 0 violations.
• Known skipped/failing check: full swiftlint --strict was run and failed on pre-existing violations outside this change set, including ios/epacUITests/AppPreviewRecordingTests.swift:12, ios/epac/Model/CabinetService.swift:27, widget/app-clip no_magic_numbers, and ios/epacTests/WorkflowSafetyTests.swift:22.
• Manual verification: cd ios && make ipad-simulator — built, installed, and launched on iPad Pro 13-inch (M5) simulator. Local screenshot captured at /tmp/epac-1978-evidence/ipad-launch-600.png.

Screenshots

• iPad simulator launch screenshot captured locally: /tmp/epac-1978-evidence/ipad-launch-600.png.

Related issue

• Closes: EPAC-1978

Release-Note: iPad and regular-width modal sheets now use a form-style presentation with medium and large detents.