feat: render an inline PDF viewer for .pdf links in chat

[X-15]

What

Renders an inline PDF viewer when an assistant message contains a link to a PDF (a URL ending in .pdf). Instead of a plain hyperlink, the message shows a preview card:

• Preview — the first page is rendered into the card.
• Tap → fullscreen — a scrollable viewer with native momentum scrolling.
• Share — from the card or the viewer, via the OS share sheet.

This is a general "Conduit can display PDFs sent in a conversation" capability; it keys purely off the .pdf link, so it works with any backend that returns/links PDFs.

How

• Adds pdfrx (PDFium) for rendering.
• No white flash: pages are pre-rasterized to images. pdfrx's live viewer paints a blank white rectangle until the async render lands, which is very visible in a debug build.
• Memory-bounded: the fullscreen viewer renders pages lazily around the viewport and holds them under a hard 64 MB LRU budget — small docs stay fully resident (flicker-free); large docs stay bounded regardless of page count.
• Caching: downloaded once into a dedicated, bounded flutter_cache_manager instance; the card pre-renders page 1 to a held image; both card and viewer open from the local file.
• Share: copies the cached file to a uniquely-named temp file (Android doesn't reliably honor share_plus fileNameOverrides) so the receiver sees a meaningful name, then sweeps old temp dirs.
• Basic resilience: retry on load failure, per-page error fallback, Semantics labels.

Notes

• New dependency: pdfrx (flutter_cache_manager / path_provider / share_plus were already present), reflected in pubspec.yaml / pubspec.lock.
• User-facing strings are hardcoded English as a starting point — happy to move them to AppLocalizations if preferred.
• Verified on Android (Samsung S23, arm64).

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