feat(wallets): in-app folder picker + roomier, full-width list controls

Wallets dialog refinements:
- More space between wallet cards (cardGap: spacingSm -> spacingMd).
- "Scan another folder" is now always visible as a full-width button (no
  expandable toggle) that opens the new picker.
- The Create-wallet row and the scan button span the full list width, so their
  right edges align with the wallet cards / Open|Import buttons.

New in-app folder picker (folder_picker.h): a Material-styled modal replacing
the raw path input. Browse the filesystem (Up / Home / click a sub-folder),
see the *.dat wallet files present + a count, and pick a folder to scan. The
picker takes over the modal surface while open (the overlay framework doesn't
nest); it remembers the last browsed location across opens. Fixed-height card
(no auto-height feedback), UTF-8-aware truncation, full-node icons confirmed
in the embedded MaterialIcons range. Added a modal-folder-picker sweep surface;
verified at 100% + 150%, dark + light.

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

src/app_sweep.cpp

@@ -404,6 +404,22 @@ void App::buildSweepCatalog()
     add("modal-console-commands", ui::NavPage::Console,
         [](App& a) { a.console_tab_.sweepSetCommandsPopup(true); },
         [](App& a) { a.console_tab_.sweepSetCommandsPopup(false); });
+
+    // In-app folder picker (Wallets → Scan another folder). Populate a small demo tree so the
+    // list shows sub-folders + wallet files, then open the wallets dialog + the picker over it.
+    add("modal-folder-picker", ui::NavPage::Settings,
+        [](App& a) {
+            std::error_code ec;
+            const std::string demo = util::Platform::getConfigDir() + "picker-demo";
+            for (const char* sub : {"Documents", "Downloads", "Backups", "wallet-archive"})
+                fs::create_directories(demo + "/" + sub, ec);
+            for (const char* f : {"wallet-cold.dat", "wallet-2023.dat"})
+                if (!fs::exists(demo + "/" + f, ec))
+                    std::ofstream(demo + "/" + f, std::ios::binary) << std::string(66000, '\0');
+            ui::WalletsDialog::show(&a);
+            ui::FolderPicker::open(demo, [](const std::string&) {});
+        },
+        [](App&) { ui::FolderPicker::close(); ui::WalletsDialog::hide(); });
 }
 
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