fix(ui): validate and clamp dialog input

Tighten input handling across several dialogs so bad/edge input can't
produce a doomed request or a confusing display:

- app_security: passphrase strength meter now factors character-class
  diversity — an all-one-class string downgrades one tier so "aaaaaaaa"
  no longer scores as high as a mixed one.
- block_info: clamp the height field to the chain tip and hide/deny
  "Next" at the tip (was only yielding a raw RPC error).
- address_label: trim surrounding whitespace before saving; a
  whitespace-only label clears it (mirrors clearing the icon).
- send_tab: re-clamp the amount when a Max send has its fee bumped in the
  confirm popup (kept the total within budget) and NUL-terminate the
  strncpy'd address/memo when a payment URI overwrites the form.
- settings_page: reject an empty/whitespace-only lite wallet path before
  dispatching an unusable open/restore request.
- peers_tab: guard ExtractIP against an empty address.
- transaction_details: disable "View in explorer" when the configured
  explorer URL is empty/whitespace so we never open a garbage link.
- app: seed-backup "Skip" now requires a second, deliberate click with a
  fund-loss warning before it dismisses.

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

src/app.cpp

@@ -2278,6 +2278,7 @@ void App::renderLiteFirstRunPrompt()
     static std::vector<std::pair<std::string, bool>> chips;      // shuffled (word, consumed)
     static int progress = 0;             // # words confirmed in order
     static double wrongFlashUntil = 0.0; // brief "not the next word" hint
+    static bool skipConfirm = false;     // "Skip" backup requires a second confirming click
     static bool creating = false;        // async create (with failover) in flight
     // Restore-from-seed step (step 3). The seed buffer is SECRET — wiped in finish() and on Back.
     static char restoreSeed[512] = {};
@@ -2305,6 +2306,7 @@ void App::renderLiteFirstRunPrompt()
         chips.clear();
         progress = 0;
         step = 0;
+        skipConfirm = false;
         creating = false;
         lite_firstrun_dismissed_ = true;
         ImGui::CloseCurrentPopup();
@@ -2415,14 +2417,28 @@ void App::renderLiteFirstRunPrompt()
                 std::mt19937 rng{std::random_device{}()};
                 std::shuffle(chips.begin(), chips.end(), rng);
                 progress = 0;
+                skipConfirm = false;
                 step = 2;
             }
             ImGui::SameLine();
             if (ui::material::TactileButton("Copy", ImVec2(80, 0))) copySecretToClipboard(seed);
             ImGui::SameLine();
-            if (ui::material::TactileButton("Skip", ImVec2(80, 0))) {
-                ui::Notifications::instance().success(TR("lite_welcome_created"), 6.0f);
-                finish();
+            if (ui::material::TactileButton(skipConfirm ? "Skip anyway" : "Skip", ImVec2(120, 0))) {
+                if (!skipConfirm) {
+                    skipConfirm = true;  // require a second, deliberate click
+                } else {
+                    ui::Notifications::instance().success(TR("lite_welcome_created"), 6.0f);
+                    finish();
+                }
+            }
+            if (skipConfirm) {
+                ImGui::Spacing();
+                ImGui::PushTextWrapPos(ImGui::GetCursorPosX() + 380.0f);
+                ImGui::PushStyleColor(ImGuiCol_Text, ui::material::Error());
+                ImGui::TextUnformatted("You have not backed up your seed — funds could be lost. "
+                                       "Skip anyway?");
+                ImGui::PopStyleColor();
+                ImGui::PopTextWrapPos();
             }
         } else if (step == 2) {
             // ── Verify: tap the words in order ──────────────────────────────────────
@@ -2471,11 +2487,24 @@ void App::renderLiteFirstRunPrompt()
             }
             if (!verified) ImGui::EndDisabled();
             ImGui::SameLine();
-            if (ui::material::TactileButton("Back", ImVec2(80, 0))) step = 1;
+            if (ui::material::TactileButton("Back", ImVec2(80, 0))) { skipConfirm = false; step = 1; }
             ImGui::SameLine();
-            if (ui::material::TactileButton("Skip", ImVec2(80, 0))) {
-                ui::Notifications::instance().success(TR("lite_welcome_created"), 6.0f);
-                finish();
+            if (ui::material::TactileButton(skipConfirm ? "Skip anyway" : "Skip", ImVec2(120, 0))) {
+                if (!skipConfirm) {
+                    skipConfirm = true;  // require a second, deliberate click
+                } else {
+                    ui::Notifications::instance().success(TR("lite_welcome_created"), 6.0f);
+                    finish();
+                }
+            }
+            if (skipConfirm) {
+                ImGui::Spacing();
+                ImGui::PushTextWrapPos(ImGui::GetCursorPosX() + 380.0f);
+                ImGui::PushStyleColor(ImGuiCol_Text, ui::material::Error());
+                ImGui::TextUnformatted("You have not backed up your seed — funds could be lost. "
+                                       "Skip anyway?");
+                ImGui::PopStyleColor();
+                ImGui::PopTextWrapPos();
             }
         } else if (step == 3) {
             // ── Restore from an existing seed phrase ────────────────────────────────