feat(custom-model): warn before launching Claude on a model too small for its own overhead

Claude Code's own fixed per-turn overhead (system prompt + tool schemas,
~36.4K tokens measured live) can exceed a small local model's entire real
context before any conversation history exists to compact — confirmed
live twice as an in:0 out:0 failure on the very first message sent.
CLAUDE_CODE_MAX_CONTEXT_TOKENS cannot fix this: it only governs when
history gets compacted, and there is none on message one.

- exceedsSafeContextFloor() (custom-model-routes.ts): true when a CLI's
  registry entry declares contextLengthVar (currently only claude) and
  the model's discovered context is below CLAUDE_MIN_SAFE_CONTEXT_TOKENS
  (40000). A no-op for every other CLI by construction.
- Both apply routes (POST /api/sessions/:id/custom-model and the
  quick-start customModel path) check this before the swap-conflict
  check and before launching/restarting anything, returning
  {requiresContextWarning, modelId, contextLength, minSafeContextTokens}
  — skipped when confirmed:true.
- Frontend: #customModelContextWarningModal + _confirmContextWarning/
  _resolveContextWarningConfirm (session-ui.js), wired into both
  _quickStartWithCustomModelConfirm and _runCustomModelEntryViaRestart
  (the path Claude actually uses) ahead of the swap-confirmation check.
  Explains the fix in-modal: give the model an explicit larger -c/
  --ctx-size in llama-swap instead of relying on --fit-ctx, which
  optimizes for the biggest model that fits rather than the biggest
  context.

Tests added for the route-level warning/confirm/skip cases and the
frontend modal + launch-flow wiring. Docs updated (custom-model-
endpoints.md, wiki/Custom-Model-Endpoints.md) and the PR's running
changeset extended.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

src/web/public/index.html

@@ -948,6 +948,29 @@
       </div>
     </div>
 
+    <!-- Custom Model Endpoint Profiles: context-window-too-small warning
+         (docs/custom-model-endpoints-plan.md) — shown before launching a CLI
+         whose own fixed system-prompt/tool-schema overhead exceeds the
+         model's real discovered context, which guarantees a first-message
+         failure regardless of CLAUDE_CODE_MAX_CONTEXT_TOKENS. See
+         _confirmContextWarning() in session-ui.js. -->
+    <div class="modal" id="customModelContextWarningModal">
+      <div class="modal-backdrop" onclick="app._resolveContextWarningConfirm(false)"></div>
+      <div class="modal-content modal-sm">
+        <div class="modal-header">
+          <h3>Context window too small</h3>
+          <button class="modal-close" onclick="app._resolveContextWarningConfirm(false)" aria-label="Cancel">&times;</button>
+        </div>
+        <div class="modal-body">
+          <p class="form-hint" id="customModelContextWarningMessage" style="white-space: pre-wrap;"></p>
+        </div>
+        <div class="modal-footer">
+          <button class="btn-toolbar" onclick="app._resolveContextWarningConfirm(false)">Cancel</button>
+          <button class="btn-toolbar btn-primary" onclick="app._resolveContextWarningConfirm(true)">Launch anyway</button>
+        </div>
+      </div>
+    </div>
+
     <!-- Cron Jobs Modal -->
     <div class="modal" id="cronModal">
       <div class="modal-backdrop" onclick="app.closeCron()"></div>