fix(components): let the conversation title head the share dialog

[lodystage]

What

In the share-conversation dialog the conversation being shared — the one thing
that identifies what you are about to publish — was the faintest line on the
screen: 12px muted, under a 15px Share conversation heading, with the
public-link notice below it running at 14px in full text-foreground.

Header (session-share-dialog.tsx):

• The conversation title now heads the panel: DialogTitle, 16px semibold
  text-foreground, truncated to one line and clear of the close button.
• Share conversation becomes the small muted label above it (12px medium),
  carried by DialogDescription.

Body (session-share-manager.tsx):

• The publicNotice line (both the plain and the globe/active shape) moves from
  text-foreground to text-muted-foreground, matching every other hint on the
  screen, so it no longer competes with the header.

No copy, structure or behaviour changes. The first screen still states that
anyone with the link can view the conversation and that images are shared while
file attachments are not; the dialog keeps its fixed header over one scrolling
body, and its accessible name is still a DialogTitle with a DialogDescription.

Before / After

	Before	After
Conversation title	12px / 400 / muted	16px / 600 / foreground (truncated)
Share conversation	15px / 600 / foreground	12px / 500 / muted
Public notice	14px / foreground	14px / muted

Rendered from the Sharing/SessionShareManager story props and checked in light
and dark, on the new-share and already-shared screens, plus a long title for the
truncation and close-button clearance.

Checks

• NODE_ENV=development vitest run on session-share-manager,
  session-share-surface, session-share-request-cards, session-access-control,
  session-share-mobile-menu — 32 passed
• tsc -p packages/components/tsconfig.json --noEmit — clean
• prettier --check on the touched files — clean

🤖 Generated with Claude Code