feat: dcrypt app icon + animated loading state

[pyramation]

Summary

Adds the dcrypt app icon and an animated loading state.

Icon (1024×1024 PNG + source SVG under apps/desktop/resources/):

• BrowserWindow({ icon: APP_ICON }) — window/taskbar icon on Linux/Windows
• app.dock?.setIcon(APP_ICON) on macOS (dev; packaged .icns comes with electron-builder config later)

Loader: the brand cube-stack SVG converted to a React Loader component (CSS keyframes: cubes drop in sequentially, 1.4s alternate). UnlockScreen now shows a full-screen loading state while unlock/create is in flight:

• creating (first-run pgpm deploy into PGlite, ~15–25s): "Setting up your encrypted database…"
• unlocking: "Unlocking your vault…" (Argon2id derivation + snapshot load)

Link to Devin session: https://app.devin.ai/sessions/04636534e07048089ffb6b78142e12cd
Requested by: @pyramation

[devin-ai-integration]

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