feat(chat): add LibreChat-inspired chat interface with MongoDB persistence

Implement full chat functionality with persistent sessions and messages
stored in MongoDB, LLM completion via Ollama (with multi-provider
dispatch support), markdown rendering, and a model selector.

- Add ChatSession/ChatMessage models with serde attributes for MongoDB
  ObjectId handling (skip_serializing_if empty, alias _id)
- Add CRUD server functions: list/create/rename/delete sessions,
  list/save messages, non-streaming chat completion
- Add raw Document collection accessor for BSON ObjectId -> String
  conversion in read paths
- Add SSE streaming endpoint (Axum handler) for future streaming support
- Add provider dispatch client (Ollama, OpenAI, Anthropic, HuggingFace)
- Add frontend components: ChatSidebar with namespace grouping,
  ChatModelSelector, ChatMessageList, ChatInputBar, ChatBubble with
  pulldown-cmark markdown rendering and StreamingBubble thinking indicator
- Rewrite ChatPage with full signal-based state management
- Add comprehensive CSS for chat UI, markdown prose, animations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -760,9 +760,11 @@ dependencies = [
 name = "dashboard"
 version = "0.1.0"
 dependencies = [
+ "async-stream",
  "async-stripe",
  "axum",
  "base64 0.22.1",
+ "bytes",
  "chrono",
  "dioxus",
  "dioxus-cli-config",
@@ -774,6 +776,7 @@ dependencies = [
  "maud",
  "mongodb",
  "petname",
+ "pulldown-cmark",
  "rand 0.10.0",
  "reqwest 0.13.2",
  "scraper",
@@ -784,10 +787,12 @@ dependencies = [
  "thiserror 2.0.18",
  "time",
  "tokio",
+ "tokio-stream",
  "tower-http",
  "tower-sessions",
  "tracing",
  "url",
+ "wasm-bindgen",
  "web-sys",
 ]
 
@@ -1127,7 +1132,7 @@ dependencies = [
  "url",
  "wasm-bindgen",
  "wasm-bindgen-futures",
- "wasm-streams",
+ "wasm-streams 0.4.2",
  "web-sys",
  "xxhash-rust",
 ]
@@ -1531,7 +1536,7 @@ dependencies = [
  "tracing",
  "wasm-bindgen",
  "wasm-bindgen-futures",
- "wasm-streams",
+ "wasm-streams 0.4.2",
  "web-sys",
 ]
 
@@ -3297,6 +3302,24 @@ dependencies = [
  "psl-types",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.12.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f86ba2052aebccc42cbbb3ed234b8b13ce76f75c3551a303cb2bcffcff12bb14"
+dependencies = [
+ "bitflags",
+ "memchr",
+ "pulldown-cmark-escape",
+ "unicase",
+]
+
+[[package]]
+name = "pulldown-cmark-escape"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "007d8adb5ddab6f8e3f491ac63566a7d5002cc7ed73901f72057943fa71ae1ae"
+
 [[package]]
 name = "quinn"
 version = "0.11.9"
@@ -3573,7 +3596,7 @@ dependencies = [
  "url",
  "wasm-bindgen",
  "wasm-bindgen-futures",
- "wasm-streams",
+ "wasm-streams 0.4.2",
  "web-sys",
  "webpki-roots 1.0.6",
 ]
@@ -3588,6 +3611,7 @@ dependencies = [
  "bytes",
  "encoding_rs",
  "futures-core",
+ "futures-util",
  "h2 0.4.13",
  "http 1.4.0",
  "http-body 1.0.1",
@@ -3610,12 +3634,14 @@ dependencies = [
  "sync_wrapper",
  "tokio",
  "tokio-rustls 0.26.4",
+ "tokio-util",
  "tower",
  "tower-http",
  "tower-service",
  "url",
  "wasm-bindgen",
  "wasm-bindgen-futures",
+ "wasm-streams 0.5.0",
  "web-sys",
 ]
 
@@ -5147,6 +5173,19 @@ dependencies = [
  "web-sys",
 ]
 
+[[package]]
+name = "wasm-streams"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9d1ec4f6517c9e11ae630e200b2b65d193279042e28edd4a2cda233e46670bbb"
+dependencies = [
+ "futures-util",
+ "js-sys",
+ "wasm-bindgen",
+ "wasm-bindgen-futures",
+ "web-sys",
+]
+
 [[package]]
 name = "wasmparser"
 version = "0.244.0"