Tighten conversation rhythm, flatten the tool list, and smooth streaming text

Conversation rhythm:
- Single `--paragraph-gap` knob drives paragraph spacing both inside a
  markdown block and between consecutive prose parts, out-specifying Tailwind
  Typography's prose margins. Code cards carry the same gap themselves so it
  holds at any Streamdown nesting depth.
- Two-tier vertical rhythm: `--turn-block-gap` separates scaffolding (tools /
  thinking) from the reply; `--tool-row-gap` keeps a tool run tight.
- Drop the prose indent so prose, tools, todos, and thinking share one left
  edge. `---` renders as quiet spacing, not a heavy rule.

Flat tool list:
- Tools always render as a standalone-row stack, never a "Tool actions · N
  steps" group. assistant-ui slices the tool range unstably (interleaved live
  vs. reconstructed-consecutive when settled), so grouping reshuffled the whole
  turn the instant it settled. Flat rows are pixel-identical either way.
- Inline approvals can no longer be buried in a collapsed group body.
- Remove the now-dead grouping helpers from tool-fallback-model.

Empty thinking:
- Suppress reasoning disclosures with no visible text (encrypted / spinner-
  coerced reasoning) instead of leaving an empty "Thinking" header.
- Tail stall indicator returns "thinking" when a running turn goes quiet.

Streaming cadence:
- Smooth character-reveal decouples visible cadence from bursty arrival.
- Flush queued text deltas before applying tool events so a tool row can't
  jump ahead of its preceding text.
- Disable Nagle on the GUI WebSocket so per-token frames aren't coalesced.

Polish: clarify/patch/vision_analyze tool meta, queue-panel + diff-lines
spacing, sticky human bubble expands on focus (not hover).

apps/desktop/src/app/session/hooks/use-message-stream.ts

@@ -410,6 +410,10 @@ export function useMessageStream({
       phase: 'running' | 'complete',
       sourceEventType?: string
     ) => {
+      // Text deltas flush on a timer but tool events apply now; flush first so
+      // a tool part can't jump ahead of the text that preceded it.
+      flushQueuedDeltas(sessionId)
+
       if (!nativeSubagentSessionsRef.current.has(sessionId)) {
         for (const subagentPayload of delegateTaskPayloads(payload, phase, sourceEventType)) {
           upsertSubagent(
@@ -428,7 +432,7 @@ export function useMessageStream({
         { pending: m => phase !== 'complete' || (m.pending ?? false) }
       )
     },
-    [mutateStream]
+    [flushQueuedDeltas, mutateStream]
   )
 
   const completeAssistantMessage = useCallback(