style: refine chat content and states

[jmagar]

Summary

Continue polishing Aurora's interactive chat block while preserving the mobile/action-rail containment fixes already on main.

Assistant content

• render assistant prose through Aurora Response instead of raw text
• add inline markdown emphasis/code and numbered citation metadata
• keep assistant turns unbubbled and suppress nested live-region announcements inside the already-live message log
• allow citation-rich turns to opt out of content-visibility containment so hover/focus previews can escape cleanly

Citations and references

• demonstrate inline citations alongside the existing compact Sources surface
• tighten Response and InlineCitation hover/focus previews to show concise hostnames instead of dumping raw URLs

Attachments

• replace the one-line preview notice with a compact, keyboard-scrollable file preview panel
• preserve the existing upload -> processing -> done lifecycle and mobile density

Conversation states

• add a compact New Chat action and an Aurora EmptyState for fresh conversations
• add /error as a local-only recoverable error demo rendered with Aurora Callout
• preserve the user's turn/attachment when the mock response fails

Composer

• add an explicit data-steering state while a response is active and the user has entered steering text
• move the base composer surface out of inline styles so streaming/steering/focus CSS states actually win

Verification

• 126/126 unit tests
• TypeScript clean
• changed-file ESLint clean
• shadcn registry build + generated-artifact freshness clean
• registry graph/capability/composition validation clean
• Storybook module bundle clean
• direct desktop/mobile Chromium flow verifies citation preview, file preview, empty/error states, steering, zero mobile overflow, and 18px mobile action rails
• added Storybook + cross-browser Playwright contracts for the new states