Research: add configurable run timeout

Surfaces the research_run_timeout_seconds setting (added in #783) in
Settings → Research as a "Max Time" field, and lets 0 disable the
wall-clock cap entirely for long deep-research runs.

- settings.py: document that 0 disables the cap; default stays 1800s.
- research_handler.py: resolve 0 (or negative) to no timeout
  (asyncio.wait_for timeout=None); other values stay bounded to
  [60, 86400] as before.
- index.html / settings.js: "Max Time" input bound to
  research_run_timeout_seconds, validated to {0} ∪ [60, 86400], with
  copy making explicit that 0 = no limit (unbounded model/API cost).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

static/index.html

@@ -1463,6 +1463,10 @@
                 <label class="settings-label">Extract Parallel</label>
                 <input id="set-researchExtractConcurrency" type="text" inputmode="numeric" placeholder="3" class="settings-select" style="width:120px;">
               </div>
+              <div class="settings-row">
+                <label class="settings-label">Max Time</label>
+                <input id="set-researchRunTimeout" type="text" inputmode="numeric" placeholder="1800 sec (0 = no limit)" class="settings-select" style="width:120px;">
+              </div>
               <div id="set-researchMsg" style="font-size:11px;color:color-mix(in srgb, var(--fg) 45%, transparent);"></div>
             </div>
           </div>