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/send_tab.cpp

@@ -1266,6 +1266,12 @@ void RenderSendTab(App* app)
     float contentStartY = ImGui::GetCursorPosY();
 
     float formAvailW = ImGui::GetContentRegionAvail().x;
+    // The compose form reads best as a centered fixed-width column, not edge-to-edge.
+    // Cap the card to a readable form width and center it; the recent-sends list below
+    // deliberately keeps the full column width (formAvailW).
+    const float sendDp = Layout::dpiScale();
+    float formCardW = std::min(formAvailW, 760.0f * sendDp);
+    float formOffsetX = std::max(0.0f, (formAvailW - formCardW) * 0.5f);
     float formW = formAvailW;
     ImGui::BeginGroup();
 
@@ -1280,7 +1286,15 @@ void RenderSendTab(App* app)
     // ================================================================
     // COMPOSE FORM — single container for all fields
     // ================================================================
+    // Full-column left edge, restored after the centered card so the recent-sends
+    // list below spans the full width again.
+    float formLeftX = ImGui::GetCursorPosX();
     {
+        // Center the capped compose card: offset the cursor by the leftover half-margin,
+        // then derive every field/divider/button from the card width (not the full column).
+        float formW = formCardW;
+        if (formOffsetX > 0.0f)
+            ImGui::SetCursorPosX(ImGui::GetCursorPosX() + formOffsetX);
         ImVec2 containerMin = ImGui::GetCursorScreenPos();
         float pad = Layout::spacingLg();
         float innerW = formW - pad * 2;
@@ -1293,8 +1307,10 @@ void RenderSendTab(App* app)
         dl->ChannelsSplit(2);
         dl->ChannelsSetCurrent(1);
 
-        // Indent content by pad so every line is inset from the card edges
-        ImGui::Indent(pad);
+        // Indent content by pad so every line is inset from the card edges. Fold in formOffsetX so the
+        // auto-layout fields land inside the centered card (Indent() positions from the window's left, so
+        // without this the fields would sit at the un-offset column while the glass card is centered).
+        ImGui::Indent(pad + formOffsetX);
         ImGui::Dummy(ImVec2(0, pad * vScale)); // top padding
 
         // ---- SOURCE ADDRESS ----
@@ -1593,7 +1609,7 @@ void RenderSendTab(App* app)
 
         // Add bottom padding
         ImGui::Dummy(ImVec2(0, pad * vScale));
-        ImGui::Unindent(pad);
+        ImGui::Unindent(pad + formOffsetX);
 
         // Enforce shared card height (matches receive tab)
         {
@@ -1626,7 +1642,8 @@ void RenderSendTab(App* app)
         }
     }
 
-    // ---- RECENT SENDS ----
+    // ---- RECENT SENDS ---- (full column width; reset X off the centered card)
+    ImGui::SetCursorPosX(formLeftX);
     RenderRecentSends(state, formW, capFont, app);
 
     ImGui::EndGroup();