feat: add 80s-style TUI bootup splash animation

[nathanhuh]

Summary

Add a short, optional-feeling TUI bootup splash that gives unic a quirky 80s terminal startup moment before landing on the context picker.

Desired Behavior

• Show a brief bootup/splash screen on TUI launch.
• Animate the unic logo or wordmark with an 80s terminal aesthetic.
• Keep the animation short and non-blocking enough that startup remains usable.
• Continue loading contexts and background checks while the splash plays where practical.
• Land on the existing context picker after the splash.

Acceptance Criteria

• TUI launch shows a short animated unic bootup screen.
• Animation advances via Bubble Tea commands rather than sleeps in render code.
• Context loading still works and the context picker appears after the splash.
• Users can skip or exit with normal quit keys.
• Tests cover boot screen progression and transition to the context picker.
• README/docs are updated if startup behavior is documented.

Notes

Requested as a quirky 80s-style bootup screen experiment on 2026-05-27.

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.

[nathanhuh]

Implemented locally on branch feat/222-bootup-splash in worktree /private/tmp/unic-222-bootup.

Summary:

• Added a short 80s-style bootup splash screen with animated unic logo reveal, scanline/progress treatment, and startup diagnostics.
• Bootup starts from Init() so existing model construction remains test-friendly.
• Contexts, update checks, and caller identity loading continue in the background while the splash is active.
• Enter, Esc, or Space skips to the context picker; q quits.
• README documents the splash and skip keys.

Validation:

• go test ./internal/app
• make test
• make build

[nathanhuh]

Updated local implementation in /private/tmp/unic-222-bootup after UX review.

Refinement:

• Boot splash now runs only when ui.last_boot_splash_version differs from the current app version, so it appears after install/update and then stays off by default.
• Added ui.boot_splash as an opt-in preference for showing the splash on every launch.
• Context picker now shows the boot splash setting and B toggles the preference interactively.
• Splash remains smooth/centered with 120ms ticks and progressive logo fill.

Validation:

• go test ./internal/app ./internal/config
• make test
• make build

[nathanhuh]

Refined the local implementation again based on settings UX feedback.

Update:

• Moved the boot splash preference out of the context picker into a dedicated Settings screen.
• Added global S shortcut to open Settings, excluding text input/filter contexts.
• Settings screen currently has a structured setting row for Boot splash, toggled with Enter/Space, with Esc/q back.
• Context picker is back to context-specific responsibilities; no one-off boot splash toggle there.
• README and help output now document Settings instead of a context-picker boot key.

Validation:

• go test ./internal/app ./internal/config
• make test
• make build