feat(ui): width-responsiveness — content max-width cap + per-surface form/input clamps

Wide/ultrawide (1440-3440px) responsiveness was unhealthy: no page/content-level
max-width cap existed, and every card/form/table derived width from raw
GetContentRegionAvail().x with floor-only clamps, so surfaces stretched edge-to-edge
(2000-3000px inputs, ballooning cards, giant grid cells, 2000px+ dead row-voids).

Root cause: adopt the (previously dead-code) clamp helpers.
- New Layout::kContentMaxWidth() (~1600dp, tunable via ui.toml [layout]
  content-max-width; <=0 disables). Cap ##ContentArea to it and center the column in
  wider windows — every tab derives from this child, so one change tames the app at
  wide widths. No-op below the cap (fills as before), so 1080p/1440p are unaffected.

Per-surface upper-clamps (std::min(cap*dp, expr), floors preserved) where a single
element is still too wide even within the capped column:
- Settings: Theme/Layout/Language combos, the font-scale slider (~3000px -> 360dp),
  the effect sliders, Explorer URL and RPC credential fields.
- Send / Receive: cap the compose / receive cards to a readable form width and center
  them (Indent(pad+offset) so the auto-layout fields align with the hand-drawn card);
  the recent-tx lists below keep the full column width.
- Chat message bubbles + composer, mining pool URL/payout inputs + stats left/right
  split, contacts search, and the lite-network add-server row / server cards / status
  panel (capped + centered).
- Wizard: vertically center the cards when they fit (was top-anchored, leaving a void
  on tall monitors), compensating the content-height measurement so it can't oscillate.

The 1600 cap also subsumes the fixed-4-column balance grids (~400px cards) and the
right-anchored row dead-gaps (voids shrink from ~2700px to ~800px), so those are left
to the cap rather than blind column/row redesigns.

Verified at 1024/1280 (and via a temporary 900dp cap to exercise the cap+center path,
since the test display clamps to 1280) across full-node + Lite + Windows (ctest green)
and an adversarial diff review (clean). The true wide/ultrawide look and the 1600dp cap
value still want eyes on a real wide monitor.

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

src/ui/windows/chat_tab.cpp

@@ -1313,7 +1313,7 @@ void RenderChatTab(App* app)
                     }
 
                     const float availW = ImGui::GetContentRegionAvail().x;
-                    const float maxBubbleW = std::max(140.0f * dp, availW * 0.72f);
+                    const float maxBubbleW = std::clamp(availW * 0.72f, 140.0f * dp, 560.0f * dp);
                     const float innerW = maxBubbleW - 2.0f * bpad;
 
                     // ── Date separator (once per calendar day): a centered pill.
@@ -1599,7 +1599,8 @@ void RenderChatTab(App* app)
             const float ringR    = std::max(7.0f, lineH * 0.42f);
             const float ringPad  = 9.0f * tdp;
             const float ringSlot = 2.0f * ringR + ringPad * 1.6f;
-            const float inputW = std::max(ringSlot + 48.0f * tdp, cw - emojiBtn - 2.0f * inGap - sendW);
+            const float inputW = std::min(720.0f * tdp,
+                                           std::max(ringSlot + 48.0f * tdp, cw - emojiBtn - 2.0f * inGap - sendW));
             const float sendX  = inputX + inputW + inGap;
             const float textW  = std::max(40.0f * tdp, inputW - ringSlot);   // input area, left of the ring
             const ImVec2 ringC(inputX + inputW - ringPad - ringR, rowY + composerBoxH - ringPad - ringR);