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/components/chat/diff-lines.tsx

@@ -38,7 +38,7 @@ export function DiffLines({ className, text, ...props }: DiffLinesProps) {
   return (
     <pre
       className={cn(
-        'mt-2 max-h-96 max-w-full min-w-0 overflow-auto rounded-md border border-border/60 bg-muted/35 px-2.5 py-1.5 font-mono text-[0.7rem] leading-relaxed text-muted-foreground',
+        'mt-1 mb-1.5 max-h-96 max-w-full min-w-0 overflow-auto rounded-md border border-border/60 bg-muted/35 px-2.5 py-1.5 font-mono text-[0.7rem] leading-relaxed text-muted-foreground',
         className
       )}
       data-slot="diff-lines"