Present local sign-in inline with color-coded states

Give the local device-flow sign-in its own page in Chopin's visual
language: a centered column on the warm ground with a light brand wash,
and one hairline card that swaps between sign-in steps in place.

The plaintext-storage consent is now an inline step in that card instead
of a modal. It takes focus on "No, cancel sign-in", and Escape still
declines. Every state is a banner colored the same way as document
callouts: neutral for progress and cancellation, warning for the vault
consent and manual fallbacks, danger for errors, and success for
completion. The one-time code shows large and monospaced like GitHub's
device page. The issue's strings are unchanged, and the hosted sign-in
page is untouched.

The page is its own scroll container, so the consent choices stay
reachable on short viewports.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: bun run ci passed: dprint, oxlint, tokens
Assistant-verification: bun run types passed: all workspaces and E2E
Assistant-verification: bun test passed: 1566 pass, 2 PostgreSQL skips, 0 fail, including the focus-geometry and shared-icon design guards
Assistant-verification: Playwright E2E passed: CI=1 locked bun run e2e, 226 passed and 5 flaky that passed on retry; local-auth passed, including inline consent decline, Escape, and accept
Assistant-verification: Playwright E2E failed then passed: the first run showed the accept button unreachable below the fold; making the page scrollable fixed it (local-auth 1/1)
Assistant-verification: agent-browser Edge manual scenario passed: idle, waiting, and cancelled states render on the live local server
User-preference: Don't use pop-up dialogs for in-flow decisions; show them inline
User-preference: Match GitHub and GitHub Next aesthetics or the existing Chopin theme; avoid off-palette colors
User-preference: Color-code UI states (info, warning, error, success) like GitHub does, with consistent copy
User-preference: Restyle only the UI a feature introduces; leave existing main UI unchanged
Co-authored-by: Alex Lavaee <lavaman131@github.com>