feat(ios): align illustrations and flow to BetterFit design system

[johnnyhuy]

Summary

Three commits that align BetterFit's iOS host app to the BetterFit design system.

• `fix(iOS): remove invalid UIApplicationSceneManifest blocking launch` — the auto-generated scene manifest declared an empty `UISceneDelegateClassName`, which left the app on a black launch screen on iOS 26 simulators. Removing it lets SwiftUI's `WindowGroup` attach directly.
• `feat(design): add identity tokens and BetterFit mark mascot` — the Ember mascot was rendered in ember orange, which the design system reserves for ordinary product actions. Adds `identity`, `identityInk`, and `identitySurface` tokens to `BFColors`, then re-renders the mascot as the BetterFit mark (three compressed yellow ovals with black features for contrast). Athlete illustrations on workout cards stay on ember product accent per the posture rule.
• `feat(ios): lead with next action and tighten Workout/Plan layouts` — Workout home collapses seven metrics into a single conclusion line; the workout preview now reads from the currently-displayed suggested workout so `Target Muscles` stays in sync with `Up Next`; the Apple Health banner collapses two icon buttons into a single tappable card; Plan padding goes from 16 to 20 and the empty-day state becomes a clear primary CTA.

Consequences

• No migrations, no schema changes, no dependency updates.
• Existing SwiftUI build (BetterFitApp) targets iOS 17+ and watchOS 10+, unchanged.
• Lint state is unchanged: 442 pre-existing violations, 25 pre-existing serious. None of these are introduced by this branch; the four files I edited are lint-clean apart from the same pre-existing violations that were there before the edit.
• `BFIllustrations.swift` is a new file in the iOS host app (Demo-only, not in the BetterFit SwiftPM library).
• `Info.plist` is gitignored (XcodeGen-generated); the corresponding `project.yml` change removes the manifest from the source-of-truth config.

Testing

• `mise run test` — 95 tests passed, 0 failures.
• `mise run ios:build:dev` — built cleanly on iPhone 16 / iOS 26.2.
• Visual verification on iPhone 16 simulator: Workout home now reads identity → conclusion → streak → next action → start CTA; Plan reads week strip → rest-day label → proper empty-state CTA; Me shows the new mark mascot at 80pt with visible face features; target muscles (Triceps / Chest / Shoulders) populate correctly from the displayed workout.
• Targeted UI regression `BetterFitAppUITests/TabNavigationUITests/testStartWorkoutButtonDoesNotOverlapContent` passes (16.9s).
• SwiftPM video pipeline checks (23/23) pass; pipeline not affected by these changes.