feat(ai-chat): realtime token counter + reasoning tokens, Claude-Code style (#151)

Tokens were only counted post-hoc (onFinish) and the header badge updated only on
chat open/switch; reasoning wasn't requested or shown. Now a counter ticks LIVE
during generation and surfaces reasoning ("thinking") tokens separately, like
Claude Code's `Thinking… · N tokens`.

Architecture (AI SDK v6): no provider gives exact per-token usage mid-stream, so
the live number is a cheap client estimate (chars/≈4) reconciled to AUTHORITATIVE
provider usage at step boundaries and turn end. The useChat per-delta re-render is
the existing realtime engine.

- server: `chatStreamMetadata` now also forwards usage on `finish-step` + `finish`;
  `sendReasoning: true`; persisted `metadata.usage` carries `reasoningTokens`
  (normalized from `outputTokenDetails` or the deprecated field).
- client: pure `count-stream-tokens` (estimateTokens / liveTurnTokens, prefers
  authoritative usage else estimate); `Thinking… · N tokens` in the typing
  indicator; collapsible "Thinking" reasoning block; throttled (~8 Hz) live
  turn-token header badge; `reasoningTokens` in types + Markdown export.

Review fixes folded in:
- v6 `finish-step.usage` is PER-STEP, not cumulative — the server now ACCUMULATES
  a running sum (new pure `accumulateStepUsage`) and sends the cumulative, which
  converges to `finish.totalUsage`, so the live counter never jumps DOWN on a
  multi-step agent turn.
- reasoning double-count: the authoritative turn-total is attributed to a block
  ONLY for a single-reasoning-part (one-step) turn; multi-step blocks each show
  their own estimate (the authoritative total stays in the header).
- no "0" badge flash at turn start (require live > 0, else show context size).
- comment refreshed (finish-step trigger).

Tests: server `accumulateStepUsage` + updated `chatStreamMetadata` (34 in the
suite); client pure-fn tests. Both tsc clean; 162 client ai-chat + the ai-chat
server suite pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/components/ai-chat.module.css

@@ -111,6 +111,24 @@
     background: light-dark(var(--mantine-color-gray-0), var(--mantine-color-dark-6));
 }
 
+/* Collapsible "Thinking" (reasoning) block: a subtle left rule, dimmer than the
+   answer so it reads as secondary thinking context above the real answer. */
+.reasoningBlock {
+    border-left: 2px solid light-dark(var(--mantine-color-gray-3), var(--mantine-color-dark-4));
+    padding-left: 8px;
+}
+
+.reasoningText {
+    margin-top: 4px;
+    font-size: var(--mantine-font-size-xs);
+    color: light-dark(var(--mantine-color-gray-7), var(--mantine-color-dark-1));
+    white-space: pre-wrap;
+}
+
+.reasoningText p {
+    margin: 0 0 4px;
+}
+
 .inputWrapper {
     flex: 0 0 auto;
     padding-top: var(--mantine-spacing-xs);