feat(welcome): redesign welcome screen with responsive layout

[puri-adityakumar]

Description

Closes #949.

Welcome screen updates:

• Gradient block wordmark: full NANOCODER on terminals ≥90 cols, compact NC monogram below that — same block font, and it never wraps
• Version line under the logo; quick-action menu and the branch/dir location line all share one full-width center axis, so narrow looks like wide
• Vertical centering on tall terminals (⅓ top, ⅔ bottom); the menu trims itself on short ones
• The boot summary is hidden while the welcome is visible (it duplicated cwd/branch/version); it still shows in run mode

Prompt input

• Spans the full terminal width at every size (40-col floor, 4-col safety margin so the rounded border never wraps) and resizes live with the window

Also fixed while in here

• Shift+Tab mode cycling no longer stacks identical [mode → model] lines: landing on normal stays silent (the status bar is the feedback) and repeated identical toasts are suppressed

One implementation note: the welcome lives in ink's <Static>, which freezes after first paint. Until the first message arrives it now renders in the live region instead, so resizing the window actually re-centers it. Once history exists it goes through Static as usual.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Demo

nanocoder-welcome.mp4

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:all completes successfully)
• Tests cover both success and error scenarios

Note: 4 git-tool tests (git-diff, getGitStatusSummarySync) fail on a clean main checkout too — they spawn temp repos and the repo's own commit-msg hook rejects their fixture commits in this environment. Unrelated to this PR.

Manual Testing

Visually verified the welcome at 38–200 cols: centered at every width, no wrapping or overflow, logo never breaks, prompt follows resizes. Provider/MCP flows are untouched by this UI-only change.

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md)

[puri-adityakumar]

Also cleaned up the Shift+Tab toast spam while testing the new welcome screen: cycling modes no longer stacks identical [normal mode → ...] green lines in the scrollback. Landing back on normal is silent now (the status bar flip is the feedback), and an identical toast won't repeat on rapid presses.

[akramcodez]

Such crazy work, @puri-adityakumar! Will review it on Monday.

[will-lamerton]

Looks crazy @puri-adityakumar - I'll leave @akramcodez to review and merge as he sees fit :)