feat: refine Android UI with muted pastel expressive design

[olavbg]

Overview

Implements the visual direction from the supplied redesign brief while keeping the existing functionality intact.

Design

• Replaces the high-saturation green/blue palette with muted pastel accents and calmer dark neutrals.
• Adds a reusable AnimatedDiagonalBackground component with a subtle two-tone diagonal sway.
• Applies the background at the theme level so it remains consistent across Timeline, Session Detail, Settings and navigation transitions.
• Uses lightly translucent surfaces so the visual identity remains present without competing with content.
• Tightens global Material 3 shape radii for cleaner, straighter geometry.
• Keeps the design deliberately restrained: readability, contrast and mobile performance remain the priority.

Existing functionality preserved

• Timeline and session detail remain the primary screens.
• Search, filters, favorites, settings and historical programme navigation are unchanged.
• The existing year picker and archive behaviour remain intact.

Verification

The changes are isolated to the shared theme/palette and reusable background component, minimizing risk to the existing Timeline and Session Detail functionality. Android build/CI should be run before merge.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep a nontransparent light background color

In light mode, this makes colorScheme.background transparent black rather than the former light color. TimelineStickyTimeHeader then calls MaterialTheme.colorScheme.background.copy(alpha = 0.95f) in TimelineScreen.kt:579, which replaces only the alpha and produces an almost opaque black header; its session-count text remains the dark onSurfaceVariant color, making that information very difficult to read. Use an opaque light background for this scheme or change the header to derive its scrim from a nontransparent surface color.

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