End‑to‑end multimodal chat with document parsing, media uploads, audio recording, and streaming markdown rendering

[SignalRT]

Summary:
This PR delivers a full multimodal chat pipeline in LLama.Web: PDF and Word document ingestion with text extraction, image and audio uploads, native in‑browser audio recording (preview/attach/discard), plus streaming response
rendering with Markdown support.

Key Features:

• Streaming chat responses rendered incrementally.
• Markdown rendering in the UI (including code blocks, lists, etc.).
• Multimodal inference pipeline with MTMD support wired into session execution.
• PDF ingestion with text extraction and truncation safeguards.
• Word (DOCX) ingestion with text extraction from document XML.
• Image uploads supported end‑to‑end (validation, storage, rendering in chat).
• Audio uploads supported end‑to‑end (validation, storage, playback in chat).
• In‑browser audio recording (MediaRecorder) with preview + attach/discard workflow.
• Capability‑aware UI (shows whether text/vision/audio are supported per model).
• Download models automatically and shows the progress

Implementation Highlights

• Attachment service handles file validation, storage, and extraction (PDF/DOCX).
• Model session builds prompts with attached media and enforces capability checks.
• Chat UI renders images/audio and guides users on supported inputs.
• Captures audio and converts it to a browser file for existing upload flow.
• Streaming tokens update the UI while Markdown is rendered on the fly.

Capability to upload images and ask about the images

Model auto-download + Capability to upload files and ask about the files