Add settings-configurable cap on logs kept in memory (ticket 026)

Settings.max_logs (default None = unlimited) is pushed to the engine
via a new EngineMessage::SetMaxLogs, enforced on every ingest and
whenever the setting changes at runtime (no restart required).
Eviction drops the oldest (lowest-sequence) logs; sequence numbering
isn't renumbered and buckets already derived from an evicted log are
left in place. Exposed via PATCH /api/settings (max_logs: number |
null) and a NavMenu numeric field (blank = unlimited).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/NavMenu.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react'
+import { useEffect, useState } from 'react'
 import type { Stats } from '../lib/api'
 import type { ColorScheme } from '../lib/api'
 import { useSettingsPanel } from '../lib/useSettingsPanel'
@@ -120,6 +120,25 @@ export function NavMenu({ stats }: { stats: Stats | null }) {
 function SettingsPanel({ panel }: { panel: ReturnType<typeof useSettingsPanel> }) {
   const [filename, setFilename] = useState('')
   const [content, setContent] = useState('')
+  const [maxLogsInput, setMaxLogsInput] = useState('')
+
+  useEffect(() => {
+    setMaxLogsInput(panel.settings?.max_logs != null ? String(panel.settings.max_logs) : '')
+  }, [panel.settings?.max_logs])
+
+  function handleMaxLogsBlur() {
+    const trimmed = maxLogsInput.trim()
+    if (trimmed === '') {
+      panel.setMaxLogs(null)
+      return
+    }
+    const parsed = Number(trimmed)
+    if (!Number.isFinite(parsed) || parsed <= 0) {
+      setMaxLogsInput(panel.settings?.max_logs != null ? String(panel.settings.max_logs) : '')
+      return
+    }
+    panel.setMaxLogs(Math.floor(parsed))
+  }
 
   async function handleFileChange(e: React.ChangeEvent<HTMLInputElement>) {
     const file = e.target.files?.[0]
@@ -158,6 +177,22 @@ function SettingsPanel({ panel }: { panel: ReturnType<typeof useSettingsPanel> }
         </div>
       </section>
 
+      <section>
+        <h3 className="mb-2 text-xs font-semibold uppercase tracking-wide text-text-subtle">Log retention</h3>
+        <div className="flex items-center gap-2">
+          <input
+            type="number"
+            min={1}
+            placeholder="Unlimited"
+            value={maxLogsInput}
+            onChange={(e) => setMaxLogsInput(e.target.value)}
+            onBlur={handleMaxLogsBlur}
+            className="w-28 rounded border border-border-soft bg-surface-1 px-2 py-1 text-xs text-text placeholder:text-text-subtle focus:border-border-strong focus:outline-none"
+          />
+          <span className="text-xs text-text-faint">max logs kept in memory (blank = unlimited)</span>
+        </div>
+      </section>
+
       <section>
         <h3 className="mb-2 text-xs font-semibold uppercase tracking-wide text-text-subtle">Import CSS</h3>
         <form onSubmit={handleImport} className="flex flex-col gap-2">